Use `wiby test` label as a trigger

[dominykas]

Implemented as described in https://github.com/pkgjs/wiby/issues/56#issuecomment-887602099

I might have to review the interactions with the checks API - something changed in Github and I was not getting the main wiby status displayed on the PR, but otherwise this works.

Some other notes to follow up:

• Does it work OK when PR is opened from a fork? Mainly does the checkout action step, which uses the github.event.pull_request.head.ref, check out the correct branch/give it the correct name? It does not #107
• Still need to figure out an option for retest Deferred to https://github.com/pkgjs/wiby/issues/109
• Still need to figure out when is a good time to do wiby clean Forgot it's covered in #96
• Probably need to remove the label if neither test nor results run Covered in https://github.com/pkgjs/wiby/issues/109

cc @zekth

[dominykas]

:tada: This PR is included in version 0.10.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: