Add pr-review e2e test and speed up e2e tests

[silverwind]

• add pr-review e2e test
• speed up most tests by logging in via POST to avoid the login form, login form is still exercised in a dedicated test
• speed up most tests be removing post-test cleanup, unnecessary because each repo is created with a unique name
• misc parallelization and api call reduction
• total suite runtime is about the same as before

---

This PR was written with the help of Claude Opus 4.7

[wxiaoguang]

Tests are still flaky

https://github.com/go-gitea/gitea/actions/runs/24878089698/attempts/1

  1) [firefox] › tests/e2e/events.test.ts:51:3 › events › logout propagation ───────────────────────

    Error: expect(locator).toBeVisible() failed

    Locator: getByRole('link', { name: 'Sign In' })
    Expected: visible
    Timeout: 20000ms
    Error: element(s) not found

    Call log:
      - Expect "toBeVisible" with timeout 20000ms
      - waiting for getByRole('link', { name: 'Sign In' })


      71 |
      72 |     // page2 should be redirected via the logout event
    > 73 |     await expect(page2.getByRole('link', {name: 'Sign In'})).toBeVisible();
         |                                                              ^
      74 |
      75 |     await context.close();
      76 |   });
        at /home/runner/work/gitea/gitea/tests/e2e/events.test.ts:73:62

    Error Context: tests/e2e-output/events-events-logout-propagation-firefox/error-context.md

  1 failed
    [firefox] › tests/e2e/events.test.ts:51:3 › events › logout propagation ────────────────────────