test: use /test for TestGiteaReTestAll (and rename it)

[chmouel]

📝 Description of the Change

The TestGiteaRetestAll E2E test was flaky due to a race condition
between the initial pull_request event and the /retest comment event.

Here is what was happening:

1. The test creates a PR, which triggers a PipelineRun
2. The test then posts /retest to re-run all pipelines
3. Both events (pull_request and retest) are processed concurrently
4. The /retest handler calls filterSuccessfulTemplates to skip
   pipelines that already succeeded
5. Because the initial PipelineRun completes very fast (it just runs
   "echo HELLOMOTO"), the retest filter sometimes sees it as already
   successful and skips it
6. This means only 1 out of 2 expected statuses are created, causing
   a timeout

The fix is to use /test instead of /retest. The /test command
re-runs all pipelines unconditionally, without checking if they
already succeeded. This eliminates the race condition entirely.

The test is also renamed from TestGiteaRetestAll to TestGiteaTestAll
to reflect the new behavior being tested, and a stale TODO comment
is removed.

🔗 Linked GitHub Issue

Fixes #

🧪 Testing Strategy

• Unit tests
• Integration tests
• End-to-end tests
• Manual testing
• Not Applicable

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[chmouel]

/retest