Creates an empty .expected file when running test output compare

[aeisenberg]

If the expected file does not already exists. This helps with test
creation and allows users to create tests more quickly.

Fixes #338.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [n/a] @github/docs-content-dsp has been cc'd in all issues for UI or other user-facing changes made by this pull request.

[adityasharad]

Sensible.
@rdmarsh2 or @dbartol do you know if we have test cases without an .expected file (rather than having an empty file)? Do consistency checks use this pattern? Just want to make sure we're not spuriously creating empty .expected files during extension use.

[rdmarsh2]

To my knowledge, we don't. All the C++ consistency checks are done with multiple query predicates imported into a single .qll, so they have predicate names in the .expected files even if they pass.

[adityasharad]

Thanks! In that case, LGTM once conflicts are resolved.