Forward multisite settings in the fork test matrix to improve CI efficiency

[lancewillett]

Scoped in PR ​https://github.com/WordPress/wordpress-develop/pull/12693

The limited-matrix-for-forks job in .github/workflows/phpunit-tests.yml declares three multisite include rows but does not forward the multisite or phpunit-config inputs to the reusable workflow.

Those rows fall back to the single-site defaults (phpunit.xml.dist), so: two exactly duplicate existing single-site jobs, and the intended multisite coverage on personal forks never runs.

The test-with-mysql, test-with-mariadb, and innovation-release callers already forward both inputs. This change makes limited-matrix-for-forks do the same, so the multisite rows load tests/phpunit/multisite.xml correctly.

Scope and impact (personal forks outside WordPress/ only; canonical matrices unchanged):

• Job count stays at 8 (no added runner load)
• Removes 2 redundant single-site jobs
• Restores 3 multisite jobs (0 effective before)
• Distinct test configurations go from 6 to 8

## 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.

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.