Add tests for scopes proposal

[takikawa]

PR #4 added WIP tests for the core spec, but it so far doesn't include any tests for any proposals, including the scopes proposal. In the future it'd be good to cover the scopes proposal.

I think it could make sense to add new JSON files for additional proposals that we support. In that case, we could rename the original tests to source-map-core-tests.json, or source-map-core-consumer-tests.json or something similar.

For scopes, the test cases would at a minimum test the scope specification format and validation for scope data. In scopes meetings, we've also discussed adding more in-depth kinds of tests that could involve, for example, stepping to a specific location in a debugger and checking which scopes are listed.