Ensures a themes template is overrideable based on the 'template' filename.
function testTemplateOverride() {
config('system.theme')
->set('default', 'test_theme')
->save();
$this
->drupalGet('theme-test/template-test');
$this
->assertText('Success: Template overridden.', 'Template overridden by defined \'template\' filename.');
}