TranslationManagerTest.php

 public function testRebuildCache()
    {
        $translator = $this->getContainer()->get('translator.default');

        $key = uniqid('TRANSLATION_KEY_', true);
        $domain = LoadTranslations::TRANSLATION_KEY_DOMAIN;
        $locale = LoadLanguages::LANGUAGE2;
        $expectedValue = uniqid('TEST_VALUE_', true) . '';

        $this->manager->saveValue($key, $expectedValue, $locale, $domain);
        $this->manager->flush();

        //Ensure that catalog still contains old translated value
        $actualValue = $translator->trans($key, [], $domain, $locale);
        $this->assertNotEquals($expectedValue, $actualValue);

        $this->manager->rebuildCache();

        //Ensure that catalog now contains new translated value
        $actualValue = $translator->trans($key, [], $domain, $locale);
        $this->assertEquals((string)$expectedValue, (string)$actualValue);
    }