Issue #1797452 by Lars Toomre: Remove t() from system tests starting with M

.../modules/system/lib/Drupal/system/Tests/Mail/MailTest.php

...modules/system/lib/Drupal/system/Tests/Menu/LinksTest.php

...ules/system/lib/Drupal/system/Tests/Menu/MenuTestBase.php

...dules/system/lib/Drupal/system/Tests/Menu/RebuildTest.php

...odules/system/lib/Drupal/system/Tests/Menu/RouterTest.php

...modules/system/lib/Drupal/system/Tests/Menu/TrailTest.php

.../system/lib/Drupal/system/Tests/Menu/TreeDataUnitTest.php

...es/system/lib/Drupal/system/Tests/Menu/TreeOutputTest.php

...system/lib/Drupal/system/Tests/Module/ClassLoaderTest.php

.../system/lib/Drupal/system/Tests/Module/DependencyTest.php

...stem/lib/Drupal/system/Tests/Module/EnableDisableTest.php

...m/lib/Drupal/system/Tests/Module/HookRequirementsTest.php

...les/system/lib/Drupal/system/Tests/Module/InstallTest.php

...s/system/lib/Drupal/system/Tests/Module/ModuleApiTest.php

.../system/lib/Drupal/system/Tests/Module/ModuleTestBase.php

...es/system/lib/Drupal/system/Tests/Module/RequiredTest.php

...s/system/lib/Drupal/system/Tests/Module/UninstallTest.php