Proposal phase process requirements for test suite

[ngzhian]

In phase 2, a test suite is added.
The entry requirement for phase 3 is:

Test suite has been updated to cover the feature in its forked repo.

But it doesn't need to run against the reference interpreter.

The test suite should run against some implementation, though it need not be the reference interpreter.

It is actually quite difficult to have tests but not have reference interpreter implementation because:

• most web engines run JS tests
• JS tests are generated from .wast files by the interpreter
• in order to get to generating JS, the test runner first runs the wast file to make sure that it passes

So in the current state, to be able to generate JS test files, you will need to implement it in the reference interpreter first.

I suggest that we make a tiny tweak to https://github.com/WebAssembly/spec/blob/main/test/core/run.py#L68 to take a flag to just generate js, and skip running the tests (and round tripping).