Initial checkin of test scripts, copied from wasi-sdk.

[sunfishcode]

This just copies in the test scripts from https://github.com/WebAssembly/wasi-sdk/tree/main/tests. We can evolve them from this point on.

[sunfishcode]

This is a first draft at a concrete setup for the testing process described in #373. I copied in the same test script that #373 describes and set up a basic directory structure, which hopefully makes it clear where things will go in the repository.

[caspervonb]

Got some things I want to change here (filters being bash scripts in particular) but I'll follow up with that.

LGTM, good to get the ball finally rolling on this.

[sbc100]

Is the idea that wasi-sdk will then get its test runner from the WASI submodule (and we can then delete its copy).?

[sunfishcode]

The wasi-sdk version of these scripts has support for running the tests with different optimization levels, and also compile-only tests, but I think what we can do is build those on top of the scripts here.

[sunfishcode]

I'm going to take the liberty of merging this, so we can proceed to iterate on top of it. If there are further comments, please file issues in the issue tracker!