mock: move subscriber mock from tracing-subscriber tests (#2369)

The `tracing-subscriber` module `tests::support` included functionality
to mock a subscriber (via the `Subscribe` trait). This code depends on
some items from `tracing_mock::collector` which should otherwise not be
public.

This change moves the mocking functionality inside `tracing-mock` behind
a feature flag. Allowing the `Expect` enum and `MockHandle::new` from
`tracing_mock::collector` to be made `pub(crate)` instead of `pub`.
Since it's now used from two different modules, the `Expect` enum has
been moved to its own module.

This requires a lot of modifications to imports so that we're not doing
wildcard imports from another crate (i.e. in `tracing-subscriber`
importing wildcards from `tracing-mock`).

Closes: #2359

tracing-mock/Cargo.toml

@@ -20,6 +20,7 @@ publish = false
 [dependencies]
 tracing = { path = "../tracing", version = "0.1.35", default-features = false }
 tracing-core = { path = "../tracing-core", version = "0.1.28", default-features = false }
+tracing-subscriber = { path = "../tracing-subscriber", version = "0.3", default-features = false, optional = true }
 tokio-test = { version = "0.4.2", optional = true }
 
 # Fix minimal-versions; tokio-test fails with otherwise acceptable 0.1.0

tracing-mock/src/expectation.rs

@@ -0,0 +1,21 @@
+use crate::{
+    event::MockEvent,
+    field,
+    span::{MockSpan, NewSpan},
+};
+
+#[derive(Debug, Eq, PartialEq)]
+pub(crate) enum Expect {
+    Event(MockEvent),
+    FollowsFrom {
+        consequence: MockSpan,
+        cause: MockSpan,
+    },
+    Enter(MockSpan),
+    Exit(MockSpan),
+    CloneSpan(MockSpan),
+    DropSpan(MockSpan),
+    Visit(MockSpan, field::Expect),
+    NewSpan(NewSpan),
+    Nothing,
+}