Issue #2160735 by Cottser, aspilicious: Add hook_theme_suggestions_alter().

core/includes/theme.inc

...b/Drupal/system/Tests/Theme/ThemeSuggestionsAlterTest.php

...ules/theme_suggestions_test/theme_suggestions_test.module

.../theme_test/lib/Drupal/theme_test/ThemeTestController.php

...e_test/templates/theme-test-general-suggestions.html.twig

...les/theme_test/templates/theme-test-suggestions.html.twig

...modules/system/tests/modules/theme_test/theme_test.module

...es/system/tests/modules/theme_test/theme_test.routing.yml

...theme-test-general-suggestions--module-override.html.twig

.../theme-test-general-suggestions--theme-override.html.twig

...mplates/theme-test-suggestions--module-override.html.twig

...emplates/theme-test-suggestions--theme-override.html.twig

core/modules/system/tests/themes/test_theme/test_theme.theme

core/modules/system/theme.api.php