`wasm-as` will do some semantic analysis on the modules, which is not
something we're looking for here.
Instead, use `wat2wasm` to generate the exact module.
This also optionally generates a test suite from the WebAssembly
testsuite, which can be enabled via passing `INCLUDE_WASM_SPEC_TESTS`
to cmake, which will generate test-wasm-compatible tests and the
required fixtures.
The generated directories are excluded from git since there's no point
in committing them.