Tests: Avoid using runInSeparateProcess where possible

[obenland]

When I first saw the @runInSeparateProcess in I thought it was a neat way to solve the defined constants problem. It does however come at a pretty big speed penalty. Let's avoid using it when we can.

Slow Tests (>= 0.2s):
  1.228s Activitypub\Tests\Integration\Test_Jetpack::test_init_skips_sync_hooks_on_wpcom
  1.210s Activitypub\Tests\Model\Test_Blog::test___construct
  1.061s Activitypub\Tests\Integration\Test_Jetpack::test_add_reader_link with data set "pending following should not have reader link"
  0.897s Activitypub\Tests\Integration\Test_Jetpack::test_add_reader_link with data set "active following with feed ID"
  0.864s Activitypub\Tests\Integration\Test_Jetpack::test_add_reader_link with data set "active following without feed ID"

From https://github.com/Automattic/wordpress-activitypub/actions/runs/18201146182/job/51820087608

Proposed changes:

• Removes runInSeparateProcess annotations and reorders tests to continue to work.

Other information:

• Have you written new tests for your changes, if applicable?

[Copilot]

Pull Request Overview

This PR removes @runInSeparateProcess annotations from PHPUnit tests to improve test execution speed, as separate processes incur significant performance overhead. The change restructures test organization while maintaining test functionality.

• Removes @runInSeparateProcess and @preserveGlobalState disabled annotations from two test methods
• Reorders test methods to ensure proper execution flow without separate processes
• Consolidates test logic while preserving all original test coverage

Comments suppressed due to low confidence (1)

tests/integration/class-test-jetpack.php:1

• The test data provider cases have been reordered, changing the execution order from 'with feed ID' first to 'without feed ID' first. This reordering should be intentional and documented, or the original order should be preserved to maintain consistency with existing test expectations.

<?php

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}