tests: Don't use deprecated assertion `assertRegExp()`

library/Icinga/Test/BaseTestCase.php

@@ -333,6 +333,23 @@ namespace Icinga\Test {
                 $adapter->exec('DROP TABLE ' . $table . ';');
             }
         }
+
+        /**
+         * Add assertMatchesRegularExpression() method for phpunit >= 8.0 < 9.0 for compatibility with PHP 7.2.
+         *
+         * @TODO Remove once PHP 7.2 support is not needed for testing anymore.
+         */
+        public static function assertMatchesRegularExpression(
+            string $pattern,
+            string $string,
+            string $message = ''
+        ): void {
+            if (method_exists(parent::class, 'assertMatchesRegularExpression')) {
+                parent::assertMatchesRegularExpression($pattern, $string, $message);
+            } else {
+                static::assertRegExp($pattern, $string, $message);
+            }
+        }
     }
 
     BaseTestCase::setupTimezone();

modules/monitoring/test/php/application/views/helpers/PluginOutputTest.php

@@ -45,7 +45,7 @@ class PluginOutputTest extends BaseTestCase
                 $html,
                 preg_quote(self::SUFFIX, '~')
             );
-            $this->assertRegExp($expect, $actual, 'Output must match example regexp');
+            $this->assertMatchesRegularExpression($expect, $actual, 'Output must match example regexp');
         } else {
             $expect = $prefix . $html . self::SUFFIX;
             $this->assertEquals($expect, $actual, 'Output must match example');

test/php/library/Icinga/File/Ini/IniWriterTest.php

@@ -275,7 +275,7 @@ inkey' => 'blarg'
         );
 
         $rendered = $writer->render();
-        $this->assertRegExp(
+        $this->assertMatchesRegularExpression(
             '~linebreak\\\\nin line~',
             $rendered,
             'newlines in values are not escaped'