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.'));
}