CI: Install PHPUnit 7.5 for WP < 5.9

See https://make.wordpress.org/core/2021/09/27/changes-to-the-wordpress-core-php-test-suite/#integration-tests-ci-changes - PHPUnit 9 removed a `PHPUnit\Util\Getopt` class, but the [fix](https://core.trac.wordpress.org/changeset/51570/) wasn't applied to WP until 5.9.

.github/workflows/integrate.yml

@@ -17,9 +17,7 @@ jobs:
         wordpress: [ '5.7', '6.2' ]
         allowed_failure: [ false ]
         include:
-          - php: '8.0'
-            # Ignore platform requirements, so that PHPUnit 7.5 can be installed on PHP 8.0 (and above).
-            composer-options: '--ignore-platform-reqs'
+          - php: '8.2'
             extensions: pcov
             ini-values: pcov.directory=., "pcov.exclude=\"~(vendor|tests)~\""
             coverage: pcov
@@ -44,10 +42,15 @@ jobs:
           ini-values: ${{ matrix.ini-values }}
           coverage: ${{ matrix.coverage }}
 
+      - name: Install PHPUnit 7.x for WP < 5.9
+        if: ${{ matrix.wordpress < 5.9 }}
+        # Ignore platform requirements, so that PHPUnit 7.5 can be installed on PHP 8.0 (and above).
+        run: composer require --dev phpunit/phpunit:"^7.5" --ignore-platform-req=php+ --no-update --no-scripts --no-interaction
+
       - name: Install Composer dependencies
         uses: ramsey/composer-install@v2
         with:
-          composer-options: "${{ matrix.composer-options }}"
+          composer-options: --ignore-platform-req=php+
 
       - name: Set up problem matchers for PHP
         run: echo "::add-matcher::${{ runner.tool_cache }}/php.json"