This change enables the running of the wasi-threads [test suite]. It
relies on a Wasmtime CLI binary being available and runs all `*.wasm`
and `*.wat` files present in the test suite directory. The results of
each execution are compared against a JSON spec file with the same base
name as the WebAssembly module. The spec file defines the expected exit
code, e.g.

This commit does not yet build any `*.c` or `*.s` files from the test
suite. That could be done later, perhaps upstream; in the meantime, this
work is still valuable as it lays the foundation for running other WASI
tests from the in-progress [wasi-testsuite] which share the same JSON
spec infrastructure.

[test suite]: https://github.com/WebAssembly/wasi-threads/tree/main/test/testsuite
[wasi-testsuite]: https://github.com/WebAssembly/wasi-testsuite

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