ws tests randomly failing

[debris]

pr: #9527
job: https://gitlab.parity.io/parity/parity-ethereum/-/jobs/100409

---- tests::ws::testing::should_not_allow_initial_connection_even_once stdout ----
thread 'tests::ws::testing::should_not_allow_initial_connection_even_once' panicked at 'called `Result::unwrap()` on an `Err` value: Error(Io(Os { code: 98, kind: AddrInUse, message: "Address already in use" }), State { next_error: None, backtrace: Some(stack backtrace:
   0:     0x55da988f23ac - backtrace::backtrace::trace::hba5a988066817d84
   1:     0x55da988f2162 - <backtrace::capture::Backtrace as core::default::Default>::default::h412d874ba4980714
   2:     0x55da988f21d8 - backtrace::capture::Backtrace::new::h54c17efd48b4adfb
   3:     0x55da983b238c - error_chain::make_backtrace::h2e7c1fce38705312
   4:     0x55da983b2414 - <error_chain::State as core::default::Default>::default::h2b81263bb968da19
   5:     0x55da983a84e4 - <jsonrpc_ws_server::error::Error as core::convert::From<ws::result::Error>>::from::h10928d47129bac08
   6:     0x55da982e5fdc - jsonrpc_ws_server::server::Server::start::hccc3794885dd7a7a
   7:     0x55da9800d9cd - parity_rpc::start_ws::h9529ebadc5614405
   8:     0x55da9821220b - parity_rpc::tests::ws::serve::h6f4918d3c656320c
   9:     0x55da982d9490 - parity_rpc::tests::ws::testing::should_not_allow_initial_connection_even_once::hec442bbec9fb5650
  10:     0x55da9836a29e - test::run_test::{{closure}}::he19c038d8c104470
                        at libtest/lib.rs:1451
                         - core::ops::function::FnOnce::call_once::hdb2fec6c62be136b
                        at /checkout/src/libcore/ops/function.rs:223
                         - <F as alloc::boxed::FnBox<A>>::call_box::h03fc71c6eb94051c
                        at /checkout/src/liballoc/boxed.rs:642
  11:     0x55da98b29609 - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:105
  12:     0x55da9838b64f - std::panicking::try::ha3567542ee7f93d3
                        at /checkout/src/libstd/panicking.rs:289
                         - std::panic::catch_unwind::he54206e207161564
                        at /checkout/src/libstd/panic.rs:392
                         - test::run_test::run_test_inner::{{closure}}::ha65c0910d1e62646
                        at libtest/lib.rs:1406
                         - std::sys_common::backtrace::__rust_begin_short_backtrace::h975948809aadadb9
                        at /checkout/src/libstd/sys_common/backtrace.rs:136
  13:     0x55da9838c004 - std::thread::Builder::spawn::{{closure}}::{{closure}}::hbf0c1fd1aa6ac94a
                        at /checkout/src/libstd/thread/mod.rs:409
                         - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::he04589d5c023d380
                        at /checkout/src/libstd/panic.rs:313
                         - std::panicking::try::do_call::h692b123353b10d66
                        at /checkout/src/libstd/panicking.rs:310
  14:     0x55da98b29609 - __rust_maybe_catch_panic
                        at libpanic_unwind/lib.rs:105
  15:     0x55da983813e6 - std::panicking::try::hdd64966142805518
                        at /checkout/src/libstd/panicking.rs:289
                         - std::panic::catch_unwind::hcacb0d4628cc5c73
                        at /checkout/src/libstd/panic.rs:392
                         - std::thread::Builder::spawn::{{closure}}::h822ad663244d6e06
                        at /checkout/src/libstd/thread/mod.rs:408
                         - <F as alloc::boxed::FnBox<A>>::call_box::h363018559453b4b8
                        at /checkout/src/liballoc/boxed.rs:642
  16:     0x55da98b0973a - <alloc::boxed::Box<(dyn alloc::boxed::FnBox<A, Output=R> + 'a)> as core::ops::function::FnOnce<A>>::call_once::h940efee7335c8f36
                        at /checkout/src/liballoc/boxed.rs:652
                         - std::sys_common::thread::start_thread::hac18b9ed82bd359d
                        at libstd/sys_common/thread.rs:24
  17:     0x55da98b07825 - std::sys::unix::thread::Thread::new::thread_start::ha15fdd35f58285a3
                        at libstd/sys/unix/thread.rs:90
  18:     0x7f603e45f6b9 - start_thread
  19:     0x7f603df7f41c - clone
  20:                0x0 - <unknown>) })', libcore/result.rs:945:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:221
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:475
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   6: rust_begin_unwind
             at libstd/panicking.rs:325
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   8: core::result::unwrap_failed
   9: parity_rpc::tests::ws::serve
  10: parity_rpc::tests::ws::testing::should_not_allow_initial_connection_even_once
  11: <F as alloc::boxed::FnBox<A>>::call_box
             at libtest/lib.rs:1451
             at /checkout/src/libcore/ops/function.rs:223
             at /checkout/src/liballoc/boxed.rs:642
  12: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:105