Group fixture checks into single Code Analysis job

[TomasVotruba]

Merge 3 separate fixture-related matrix entries in code_analysis.yaml into one Fixture Checks job. Fewer separate GitHub checks, same coverage.

Before — 3 checks:

• Check before/after test fixture on no-changes
• Check fixture classes are different to nodes
• Check no "*.php" files in rules Fixture directory

After — 1 check Fixture Checks:

-
    name: 'Fixture Checks'
    run: |
        php scripts/check-before-after-same-fixtures.php
        php scripts/avoid-short-node-names-in-fixtures.php
        php scripts/no-php-file-in-fixtures.php

Note: if branch protection lists the old 3 check names as required, update required checks to Fixture Checks.