tests(config): add unit tests from legacy config

[adamraine]

This PR adds a bunch of test cases I found in the legacy/config-test.js that were not present in any of our new test case files:

• config-test.js (not legacy)
• config-helpers-test.js
• validation-test.js
• filters-test.js

There are some minor missing features from the legacy config, but this PR is only going to address test coverage. The missing features should be addressed in separate PRs. For reference, the missing features are:

• Throw if a requiredArtifacts is not in the config
• throws for invalid use of skipAudits and onlyAudits
• Deduplicate artifacts on id (previously done by gatherer name)

#15060

[adamraine]

I'm gonna temporarily skip the legacy/config-test.js stuff to see where we are with codecov.

[adamraine]

Codecov is being weird, but it looks like we don't lose any coverage when skipping the legacy config tests (excluding a bunch of stuff in legay/config/config.js which was expected)

I'm going to unskip the tests, and this should be landable.

[connorjclark]

Happy we won't be losing this test coverage :)