Add more in-depth tests for debuggers

[takikawa]

Right now the source map spec tests mostly test conformance with the source map format, along with some basic mapping semantics properties.

It'd be nice to also test more in-depth debugging properties, such as the effect of ignoreList on debugger stepping, or the effects of scopes from the scope proposal on stepping.

I think it'd make sense to put these in a separate JSON file, as some tools (e.g., the validator) won't support these test cases.