Build/test tools: forward multisite settings in the fork test matrix to improve CI efficiency

[lancewillett]

Trac ticket

https://core.trac.wordpress.org/ticket/65712

What

The limited-matrix-for-forks job in phpunit-tests.yml declares multisite include rows but never forwards multisite or phpunit-config to the reusable workflow. Those rows fall back to single-site defaults, so each one duplicates an existing single-site job instead of exercising multisite.

This forwards both inputs so the multisite rows select tests/phpunit/multisite.xml, matching the test-with-mysql, test-with-mariadb, and innovation-release callers.

Why

The fork matrix currently spends jobs on duplicate single-site runs and skips the multisite coverage it intends to provide. The fix restores that coverage without adding or removing any jobs.

Expected impact

Scope: the limited-matrix-for-forks job only. It runs on personal forks outside the WordPress/ organization, so the canonical matrices are untouched.

• Job count: 8 before, 8 after. No change to runner load or CI minutes.
• Redundant jobs: 2 before, 0 after. Two include rows duplicated existing single-site jobs; they now run multisite.
• Multisite jobs: 0 effective before, 3 after. The three multisite: true rows now load tests/phpunit/multisite.xml.
• Distinct test configurations: 6 before, 8 after.

A healthy run shows three jobs configured with tests/phpunit/multisite.xml and no increase in total job count.

Definition of done

A reviewer can validate and verify the change as follows:

• The diff forwards multisite and phpunit-config in limited-matrix-for-forks only, matching the test-with-mysql caller.
• actionlint passes on phpunit-tests.yml.
• On a fork run, the three multisite jobs show tests/phpunit/multisite.xml as the PHPUnit config; the single-site jobs still show phpunit.xml.dist.
• The two formerly-duplicate jobs now differ from their single-site counterparts at the same PHP and database version.
• No change to test-with-mysql, test-with-mariadb, or test-innovation-releases.

Testing

• actionlint passes on the changed workflow.
• The three affected include rows resolve to tests/phpunit/multisite.xml; single-site rows are unchanged.
• This matrix runs only on personal forks outside the WordPress/ organization, so it does not affect the canonical CI.

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[Copilot]

Pull request overview

This PR fixes the fork-only PHPUnit workflow matrix so that multisite matrix rows actually run as multisite when calling the reusable PHPUnit workflow, restoring the intended coverage without increasing job count.

Changes:

• Forward multisite from the limited-matrix-for-forks job matrix into the reusable workflow inputs.
• Forward phpunit-config, selecting tests/phpunit/multisite.xml when multisite is enabled (otherwise phpunit.xml.dist), aligning behavior with other PHPUnit workflow callers.

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[lancewillett]

For reviewers: this PR changes the limited-matrix-for-forks job which by design runs only on personal forks (its if excludes the WordPress/ org), so this PR's own CI doesn't exercise it.

It forwards multisite and phpunit-config to the reusable workflow, matching the test-with-mysql and test-with-mariadb callers. The 3 multisite: true include rows now load tests/phpunit/multisite.xml instead of falling back to single-site, restoring multisite coverage without adding or removing jobs.