Ensures a theme's template is overrideable based on the 'template' filename.
function testTemplateOverride() {
config('system.theme')
->set('default', 'test_theme_phptemplate')
->save();
$this
->drupalGet('theme-test/template-test');
$this
->assertText('Success: Template overridden with PHPTemplate theme.', t('Template overridden by PHPTemplate file.'));
}