Check for missing lang strings in a similar way to how we check for missing privacy providers

[brendanheywood]

eg this plugin passed plugin ci, but then failed in the context of a whole repo:

So I want to run core_cache\administration_helper_test::test_get_summaries in plugin ci, but only for this plugin (depending on how long it takes)

There was 1 error:

1) core_cache\administration_helper_test::test_get_summaries
Unexpected debugging() call detected.
Debugging: String does not exist. Please check your string definition for cachedef_cms_content_course/mod_cms
* line 163 of /lib/classes/lang_string.php: call to debugging()
* line 612 of /cache/classes/definition.php: call to core\lang_string->__construct()
* line 191 of /cache/classes/administration_helper.php: call to core_cache\definition->get_name()
* line 98 of /cache/tests/administration_helper_test.php: call to core_cache\administration_helper::get_definition_summaries()
* line 1617 of /vendor/phpunit/phpunit/src/Framework/TestCase.php: call to core_cache\administration_helper_test->test_get_summaries()
* line 1223 of /vendor/phpunit/phpunit/src/Framework/TestCase.php: call to PHPUnit\Framework\TestCase->runTest()
* line 76 of /lib/phpunit/classes/advanced_testcase.php: call to PHPUnit\Framework\TestCase->runBare()
* line 729 of /vendor/phpunit/phpunit/src/Framework/TestResult.php: call to advanced_testcase->runBare()
* line 973 of /vendor/phpunit/phpunit/src/Framework/TestCase.php: call to PHPUnit\Framework\TestResult->run()
* line 685 of /vendor/phpunit/phpunit/src/Framework/TestSuite.php: call to PHPUnit\Framework\TestCase->run()
* line 685 of /vendor/phpunit/phpunit/src/Framework/TestSuite.php: call to PHPUnit\Framework\TestSuite->run()
* line 685 of /vendor/phpunit/phpunit/src/Framework/TestSuite.php: call to PHPUnit\Framework\TestSuite->run()
* line 651 of /vendor/phpunit/phpunit/src/TextUI/TestRunner.php: call to PHPUnit\Framework\TestSuite->run()
* line 146 of /vendor/phpunit/phpunit/src/TextUI/Command.php: call to PHPUnit\TextUI\TestRunner->run()
* line 99 of /vendor/phpunit/phpunit/src/TextUI/Command.php: call to PHPUnit\TextUI\Command->run()
* line 107 of /vendor/phpunit/phpunit/phpunit: call to PHPUnit\TextUI\Command::main()
* line 122 of /vendor/bin/phpunit: call to include()