Add automated tests

Further work is needed - tests do not yet add exemptions.

It's also unclear why setting site into maintenance mode, then calling $this->assertSession() causes the tests to fail with the following issue

1) Drupal\Tests\maintenance_exempt\Functional\ExemptionsTest::testNodesNotAccessible
Exception: Deprecated function: preg_quote(): Passing null to parameter #1 ($str) of type string is deprecated
Drupal\Core\Path\PathMatcher->matchPath()() (Line: 82)


/var/www/html/web/core/lib/Drupal/Core/Test/HttpClientMiddleware/TestHttpClientMiddleware.php:55
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:209
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:158
/var/www/html/vendor/guzzlehttp/promises/src/TaskQueue.php:52
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:251
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:227
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:272
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:229
/var/www/html/vendor/guzzlehttp/promises/src/Promise.php:69
/var/www/html/vendor/guzzlehttp/guzzle/src/Client.php:189
/var/www/html/web/core/tests/Drupal/Tests/DrupalTestBrowser.php:139
/var/www/html/vendor/symfony/browser-kit/AbstractBrowser.php:369
/var/www/html/vendor/behat/mink-browserkit-driver/src/BrowserKitDriver.php:122
/var/www/html/vendor/behat/mink/src/Session.php:175
/var/www/html/web/core/tests/Drupal/Tests/UiHelperTrait.php:275
/var/www/html/web/modules/contrib/maintenance_exempt/tests/src/Functional/ExemptionsTest.php:86

ERRORS!
Tests: 2, Assertions: 8, Errors: 1.

OK, this was due to an actual bug in the module, see commit https://git.drupalcode.org/project/maintenance_exempt/-/commit/2db4a3c32...

Adding more tests.

Further work needed.

• testNodeExemptedByPath() works locally but not on CI.
• testNodeExemptedByIpAddress() does not work at all - need a way to predict the IP address that a call to drupalGet() in a browser test will be received from, so we can put it in the maintenance exempt config.

Also, phpunit is mandatory, but phpunit (next major) is not. We should consider making it so.

testNodeExemptedByIpAddress() now works locally but not in CI.