test: improve lib/internal/source_map/source_map.js coverage
This improves a test coverage in lib/internal/source_map/source_map.js
coverage
Ref: https://coverage.nodejs.org/coverage-0699150267c08fb2/lib/internal/source_map/source_map.js.html#L154 Ref: https://coverage.nodejs.org/coverage-0699150267c08fb2/lib/internal/source_map/source_map.js.html#L165
This validates that index source map is supported.