Convert multi-memories .wasm test files to .wast

[ashleynh]

@ashleynh

I added the multi-memories wasm test files to verify correct parsing of binary format and printing to text format.

Makes sense, we do want that coverage. But adding a wast file in the same directory should get the same test coverage (it will also emit .fromBinary outputs etc. - it will write to binary, read, then print that). And text tests are generally easier to handle.

We do have some actual .wasm files in the test suite, but mainly to handle code that can't be directly represented in the text format for some reason.

Originally posted by @kripken in #4811 (comment)