MDL-66903 testing: Add support for a \tests\ namespace during tests

This commit:
- introduces a \tests\ sub-namespace for use in unit tests only
- the path to this the tests/classes directory of the owning parent
- files here are excluded from unit test runs

This is agreed per policy in MDL-80855.

lib/classes/component.php

@@ -172,6 +172,38 @@ class core_component {
             require($file);
             return;
         }
+
+        if (PHPUNIT_TEST) {
+            // For unit tests we support classes in `\frankenstyle_component\tests\` to be loaded from
+            // `path/to/frankenstyle/component/tests/classes` directory.
+            // Note: We do *not* support the legacy `\frankenstyle_component_tests_style_classnames`.
+            if ($component = self::get_component_from_classname($classname)) {
+                $pathoptions = [
+                    '/tests/classes' => "{$component}\\tests\\",
+                    '/tests/behat' => "{$component}\\behat\\",
+                ];
+                foreach ($pathoptions as $path => $testnamespace) {
+                    if (preg_match("#^" . preg_quote($testnamespace) . "#", $classname)) {
+                        $path = self::get_component_directory($component) . $path;
+                        $relativeclassname = str_replace(
+                            $testnamespace,
+                            '',
+                            $classname,
+                        );
+                        $file = sprintf(
+                            "%s/%s.php",
+                            $path,
+                            str_replace('\\', '/', $relativeclassname),
+                        );
+                        if (!empty($file) && file_exists($file)) {
+                            require($file);
+                            return;
+                        }
+                        break;
+                    }
+                }
+            }
+        }
     }
 
     /**