Re-use of interpreter tests for the wasm parser

[ColinEberhardt]

Following on from our conversations on slack ...

• it would be ideal to re-use some of the existing tests for the wasm decoder / gen
• binary wasm files are not great test cases
• the wat files in the wast-parser/test/fixtures are fragments, and are not valid wat files (perhaps rename to something like .watf to make this clear?)

To test the wasm-parser I'd like to re-use of the wast (i.e. wat) files within packages/webassemblyjs/test/fixtures/, using wat2wasm to convert them to wasm, then using the wasm-parser to convert the wasm into AST, this can then be compared with the AST generated by wast-parser.

I've had a go at this ... here is what happens to far. For 36 test files ...

A number are not actually valid wat files:

 'packages/webassemblyjs/test/fixtures/parsing-invalid-index/module.wast',
  'packages/webassemblyjs/test/fixtures/instantiate-data-section/global-offset/module.wast',
  'packages/webassemblyjs/test/fixtures/importObject/module.wast',
  'packages/webassemblyjs/test/fixtures/if-type-inference-incorrect-result/module.wast',
  'packages/webassemblyjs/test/fixtures/if-type-inference-incorrect-branches/module.wast',
  'packages/webassemblyjs/test/fixtures/global-type-mismatch/module.wast',
  'packages/webassemblyjs/test/fixtures/export-global-i32-mut/module.wast',
  'packages/webassemblyjs/test/fixtures/duplicate-export-name/module.wast',
  'packages/webassemblyjs/test/fixtures/call-func-with-i64-result/module.wast',
  'packages/webassemblyjs/test/fixtures/validate-func-result-type/module.wast',
  'packages/webassemblyjs/test/fixtures/trap-throws-runtime-error/module.wast' ]

There are a couple of issues here.

1. Ones where the wat has simple validation errors, e.g. elements in the wrong order.
2. Ones that are not valid because that is the nature of the test.

For (1) I'd propose fixing the wat file, and updating wast-parser with additional validation. For (2) I think these should be moved into wast-parser.

Of the remaining, comparing wast => AST vs. wasm => AST, shows some metadata differences, e.g. loc and other metadata. This should be easy to strip out allowing a comparison.