workflows: test improvements

[wikkyk]

Issue #, if available:
Follow-up to #384 #390

Description of changes:
I converted the test workflows into a reusable workflow to reduce duplication.
I amended the comment message to also mention test runs requiring approval and reworded the comment text to make it terser.

[github-actions]

External PR

Test runs on external PRs require manual approval.

Note: This PR changes the following non-go, non-docs files:
.github/workflows/comment.yml
.github/workflows/e2e-comment.yml
.github/workflows/run-tests.yml
.github/workflows/test-comment.yml
.github/workflows/test-external.yml
.github/workflows/test.yml

[github-actions]

🚀 e2e tests run

We add labels to the PRs to control the e2e test runs by running specific tests and skipping some test contexts,
please follow this guide:

Label	Behaviour
none	Run Generic tests
e2e/none skip all e2e tests (documentation etc) - overrides all e2e/* labels	Do not run any tests (overrides all e2e/ flags)
e2e/flatcar run Flatcar e2e tests	Add Flatcar tests to the run

ℹ️ Ask a Member to add the requested labels if you don't have enough permissions.

[github-actions]

External PR

Test runs on external PRs require manual approval.

Note: This PR changes the following non-go, non-docs files:
.github/workflows/comment.yml
.github/workflows/e2e-comment.yml
.github/workflows/run-tests.yml
.github/workflows/test-comment.yml
.github/workflows/test-external.yml
.github/workflows/test.yml

[github-actions]

External PR

Test runs on external PRs require manual approval.

Note: This PR changes the following non-go, non-docs files:
.github/workflows/comment.yml
.github/workflows/e2e-comment.yml
.github/workflows/run-tests.yml
.github/workflows/test-comment.yml
.github/workflows/test-external.yml
.github/workflows/test.yml