Tests: Avoid repeated user creation in WP_Test_REST_Post_Meta_Fields

[SainathPoojary]

Optimizes fixture setup in WP_Test_REST_Post_Meta_Fields by avoiding repeated editor user creation across individual tests.

Previously, grant_write_permission() called $factory->user->create() each time it was invoked (65+ calls across test methods and data provider iterations). This introduced unnecessary database insertions and teardown overhead for tests that only needed an authenticated editor user.

This change creates a shared editor user once in wpSetUpBeforeClass(), sets that user in grant_write_permission(), and cleans it up via self::delete_user() in wpTearDownAfterClass(). All 151 tests and 581 assertions remain unchanged and pass.

Performance

Measured locally across three consecutive runs comparing trunk to this branch:

Run	Trunk Test Time (junit.xml)	Trunk Wall Time	Fix Test Time (junit.xml)	Fix Wall Time
Run 1	2.386s	2.695s	1.829s	2.287s
Run 2	2.295s	2.612s	1.802s	2.219s
Run 3	2.242s	2.548s	1.850s	2.325s
Average	2.308s	2.618s	1.827s	2.277s

Trac ticket: https://core.trac.wordpress.org/ticket/66074

Use of AI Tools

AI assistance: Yes
Tool(s): Github Copilot
Used for: Edge-case checks and assistance with the PR description, implementation and testing were done and reviewed by me.

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[SainathPoojary]

I noticed a few of the CI test jobs are failing, so I spent some time investigating. I ran the tests locally, but they are passing for me without any issues.

I also checked trunk and a few other open PRs, and it looks like these similar failures are happening there as well. Since this is happening other places as well, it seems like these might be random test failures rather than something caused by these changes.