Move shared tests to separate files

[sunfishcode]

Currently, this threads proposal repo adds shared memory tests to existing test files in test/core: memory.wast, imports.wast, and exports.wast. For example:

https://github.com/WebAssembly/threads/blob/main/test/core/memory.wast#L9

However, not all wasm engines are expected to implement this threads proposal in its current form, even once it's standardized, some because of the nondeterminism, and some because of the effective dependency on Web Worker-style threads.

Would it make sense to move the tests for shared memory into separate files, so that engines which don't implement shared memories can easily filter them? This would be similar to the SIMD tests, which are all named simd_*.wast.

[rossberg]

I agree, and would go even further. As with the simd tests, we should have a subdirectory threads, and all related tests should go there. Similarly for other non-trivial and possibly profile-dependent feature proposals, like exceptions, gc, continuations.