unloadFiles throws MODULE_NOT_FOUND in ESM context for files added with relative path

[ctjlewis]

Prerequisites

• Checked that your issue hasn't already been filed by cross-referencing issues with the faq label
• Checked next-gen ES issues and syntax problems by using the same environment and/or transpiler configuration without Mocha to ensure it isn't just a feature that actually isn't supported in the environment in question or a bug in your code.
• 'Smoke tested' the code to be tested by running it outside the real test suite to get a better sense of whether the problem is in the code under test, your usage of Mocha, or Mocha itself
• Ensured that there is no discrepancy between the locally and globally installed versions of Mocha. You can find them with: node node_modules/.bin/mocha --version(Local) and mocha --version(Global). We recommend that you not install Mocha globally.

Description

Programmatically using mocha.addFile(file) and mocha.unloadFiles() in an ES module context throws MODULE_NOT_FOUND when mocha.addFile(...) is called with a relative path.

Wrapping the module resolution logic with path.resolve(...) resolves this issue by ensuring the path is absolute.

Steps to Reproduce

https://repl.it/@christiantjl/MochaFileUnloader#fails.js
node succeeds.js shows that an absolute path works without issue, and node fails.js shows that adding with a relative path throws an error.

[ctjlewis]

This can be worked around for the time being by only adding absolute filepaths to Mocha:

mocha.addFile(path.resolve(file))