GH Actions: fix running of the tests

While the builds currently all show "green", if you actually fold out the "PHPUnit Tests" step, you can see that the tests haven't actually been running on PHP 5.3, 5.4, 5.5, 5.6.

As of PHP 7.0, they have been running, but running against a mismatched PHPUnit version as all test runs are run against PHPUnit 9.5.x, while PHPUnit 9.5 is officially only supported on PHP 7.3 and higher.

Additionally, PHPUnit was being installed 3 (!) times, once via the `setup-php` action, once via the `composer install` and once via the `php-actions/phpunit` action.

To ensure that the tests are always run against the most appropriate PHPUnit version for the PHP version against which the tests are being run, I propose to:
* Remove the installing of PHPUnit via `setup-php`.
* Remove the use of the `php-actions/phpunit` action.
* Defer to the Composer installed PHPUnit version in all cases.

I'm also removing the explicit ini settings for the "moderate" and "modern" jobs.
These look like they were copied over from example code, but these values don't have any effect on the test runs in these workflows, so may as well be removed.

For the "low" job, however, having some ini values set prevents the tests erroring out too quickly.

.github/workflows/ci.yml

@@ -10,7 +10,6 @@ jobs:
       matrix:
         operating-system: ['ubuntu-18.04']
         php-versions: ['5.3', '5.4', '5.5', '5.6', '7.0']
-        phpunit-versions: ['7.5.20']
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -20,8 +19,7 @@ jobs:
         with:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl
-          ini-values: post_max_size=256M, max_execution_time=180
-          tools: phpunit:${{ matrix.phpunit-versions }}
+          ini-values: max_execution_time=600, memory_limit=256M
           coverage: none
 
       - name: Use Composer 1.x
@@ -31,9 +29,7 @@ jobs:
         uses: "ramsey/composer-install@v1"
 
       - name: PHPUnit tests
-        uses: php-actions/phpunit@v2
-        with:
-          memory_limit: 256M
+        run: vendor/bin/phpunit
 
   moderate:
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.operating-system }}
@@ -42,7 +38,6 @@ jobs:
       matrix:
         operating-system: ['ubuntu-latest']
         php-versions: ['7.1', '7.2', '7.3']
-        phpunit-versions: ['latest']
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -52,18 +47,13 @@ jobs:
         with:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl, sodium
-          ini-values: post_max_size=256M, max_execution_time=180
-          tools: phpunit:${{ matrix.phpunit-versions }}
           coverage: none
 
       - name: Install Composer dependencies
         uses: "ramsey/composer-install@v1"
 
       - name: PHPUnit tests
-        uses: php-actions/phpunit@v2
-        timeout-minutes: 30
-        with:
-          memory_limit: 256M
+        run: vendor/bin/phpunit
 
   modern:
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.operating-system }}
@@ -72,7 +62,6 @@ jobs:
       matrix:
         operating-system: ['ubuntu-latest']
         php-versions: ['7.4', '8.0']
-        phpunit-versions: ['latest']
     steps:
       - name: Checkout
         uses: actions/checkout@v2
@@ -82,15 +71,10 @@ jobs:
         with:
           php-version: ${{ matrix.php-versions }}
           extensions: mbstring, intl, sodium
-          ini-values: post_max_size=256M, max_execution_time=180
-          tools: phpunit:${{ matrix.phpunit-versions }}
           coverage: none
 
       - name: Install Composer dependencies
         uses: "ramsey/composer-install@v1"
 
       - name: PHPUnit tests
-        uses: php-actions/phpunit@v2
-        timeout-minutes: 30
-        with:
-          memory_limit: 256M
+        run: vendor/bin/phpunit