Add npm run clean-test-dir

[robertbrignull]

Sometimes the tests can fail to run and cleaning the .vscode-test dir is necessary to fix them. This PR adds this as a npm script so it's easier to remember that this option exists and what to do.

Is using rm and this path ok? Will this be ok for all operating systems?

Alternative suggestions for naming are also welcome.

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.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[norascheuch]

Looks good, thank you! Would it be useful to mention this in the testing.md file?

[koesie10]

Sometimes, it seems like there are also .vscode-test directories in subdirectories (like in test/vscode-tests/no-workspace). Could we also remove those automatically when running this?

[robertbrignull]

I've switched it to use find to locate and delete all cases of .vscode-test directories.

I haven't tested it but the info I've seen says these args should work on linux as well as mac, and therefore hopefully windows too in a gitbash or WSL shell.

[koesie10]

Thanks!