Fix --config loading: cwd-relative, fallback to require()

[cspotcode]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

Fixes #3822.

--config is first interpreted as a cwd-relative path. We check if it exists and is a file. (not directory)
If found, we try to parse it according to its file extension. (this behavior is unchanged)
If not found, we attempt to require.resolve() it relative to cwd.
If require.resolve() is able to locate a file, we require() it. We do not check file extension nor use a custom loader.

Error messages are logged for a) failing to locate a file in the first place and b) failing to parse or load the file that was found. For b) the error message logs the resolved path, so users can see that we found e.g. ../../node_modules/monorepo-shared-config/mocha.js.

---

Worth noting, cwd may not always be the correct directory for resolving. For example, if in the future the --config value comes from a package.json file -- "mocha": "@cspotcode/my-mocha-config" -- then we should resolve relative to the directory containing package.json, not necessarily cwd.

Alternate Designs

Why should this be in core?

Benefits

Possible Drawbacks

Applicable issues

#3822
#3823
#3829

[coveralls]

Coverage increased (+0.03%) to 91.738% when pulling 523dcac on cspotcode:fix-config-loading into b079d24 on mochajs:master.

[cspotcode]

Added tests.

[juergba]

The --config loading should be identical for all RC file extensions, so the status-quo doesn't make sense. You are right.
@cspotcode are you still interested in this PR?

We could try to get this one in somehow. Since lib/utils.js is part of the browser bundle, we should not put your helper function into this module.

[cspotcode]

IIRC the discussions for #3822, #3823 and #3829 left a bad taste in my mouth, so I'd rather someone else finish this one.

[juergba]

@cspotcode ok. Our team has changed meanwhile, but thanks anyway. I remember times when I was sweating blood because of this Windows CRLF publishing story.
Closing.