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`.
* Run the availability tests as well as the _real_ tests against `lowest` and `stable` for PHPUnit 9 on PHP 7.3 - 8.0.

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 - 8.0 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.x range.

Also:
* As the coding standards package has a minimum PHP requirement of PHP 7.2, it would block the `composer install` for lower PHP versions to run the tests, so removing the package for this workflow.

Author: jrfnl

.github/workflows/run-tests.yml

@@ -21,11 +21,21 @@ jobs:
 
     strategy:
       matrix:
-        php: ['8.0', '7.4', '7.3']
-        dependency-version: [prefer-lowest, prefer-stable]
+        php: ['8.0', '7.4', '7.3', '7.2', '7.1', '7.0', '5.6', '5.5', '5.4']
+        dependency-version: ['prefer-stable']
         experimental: [false]
 
         include:
+          - php: '7.3'
+            dependency-version: 'prefer-lowest'
+            experimental: false
+          - php: '7.4'
+            dependency-version: 'prefer-lowest'
+            experimental: false
+          - php: '8.0'
+            dependency-version: 'prefer-lowest'
+            experimental: false
+
           - php: '8.1'
             dependency-version: 'prefer-stable'
             experimental: true
@@ -44,6 +54,15 @@ jobs:
         coverage: none
         tools: composer
 
+    # Remove the coding standards package as it has a higher minimum PHP
+    # requirement and would prevent running the tests on older PHP versions.
+    - name: 'Composer: remove CS dependency'
+      run: composer remove --dev --no-update dms/coding-standard
+
+    - name: 'Composer: update PHPUnit for testing lowest'
+      if: ${{ matrix.dependency-version == 'prefer-lowest' }}
+      run: composer require --no-update phpunit/phpunit:"^9.0"
+
     - name: Install dependencies - normal
       if: ${{ matrix.php < 8.1 }}
       run: |