GH Actions: adjust the test workflow

... to:
* Run the availability tests against all PHP versions on which the package can be installed with `prefer-stable` (`highest`).
* Run the availability tests, as well as the _real_ tests against `lowest` and `stable` for PHPUnit 9.1+ on PHP 7.3 - 8.1.

The test selection is done automatically via the `phpunit.xml.dist` file as adjusted in the previous commit.

As for the running of the _real_ tests against the `lowest` supported PHPUnit 9 version, this requires some tweaking to the workflow script.
As PHPUnit didn't always have an explicit requirement for PHP, running with `prefer-lowest` on PHP 7.3 - 8.1 with the current PHPUnit version constraints would install PHPUnit 4.8.
So, for the `prefer-lowest` runs, we need to make sure that `lowest` is correctly interpreted as "lowest" within the PHPUnit 9.1+ range.

Author: jrfnl

.github/workflows/ci.yml

@@ -34,9 +34,20 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php: ['7.3', '7.4', '8.0', '8.1']
+        php: ['5.4', '5.5', '5.6', '7.0', '7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
+        dependency-versions: ['highest']
 
-    name: PHP ${{ matrix.php }}
+        include:
+          - php: '7.3'
+            dependency-versions: 'lowest'
+          - php: '7.4'
+            dependency-versions: 'lowest'
+          - php: '8.0'
+            dependency-versions: 'lowest'
+          - php: '8.1'
+            dependency-versions: 'lowest'
+
+    name: PHP ${{ matrix.php }} - ${{ matrix.dependency-versions }}
 
     steps:
       - name: Checkout code
@@ -49,10 +60,16 @@ jobs:
           ini-values: zend.assertions=1, error_reporting=-1, display_errors=On
           coverage: xdebug
 
+      - name: 'Composer: update PHPUnit for testing lowest'
+        if: ${{ matrix.dependency-versions == 'lowest' }}
+        run: composer require --no-update phpunit/phpunit:"^9.1"
+
       # Install dependencies and handle caching in one go.
       # @link https://github.com/marketplace/actions/install-composer-dependencies
       - name: Install Composer dependencies
         uses: "ramsey/composer-install@v1"
+        with:
+          dependency-versions: ${{ matrix.dependency-versions }}
 
       - name: Execute Unit Tests
         run: vendor/bin/phpunit --coverage-text