Since you are looking for an official certificate, you should check the conditions of the Certificate Examinations in Polish as a Foreign Language / Egzaminy Certyfikatowe z Języka Polskiego jako Obcego. As with other official examinations, the rules are strict: exams take place on specific dates and under specific conditions that require physical presence of the candidate. See the Examination Regulations, which specify, for example, the following:
- Candidates must arrive in the examination room on time, in accordance with the announced examination schedule.
- Candidates enter the examination room in a prescribed order at the set time. Each candidate should hold an ID with a photograph and produce it at the entrance to the examination room.
Just these first two conditions are incompatible with online testing, because it becomes too easy to get help from somebody else, etc.
When you look at online tests, they probably can't be considered as "official" (except Profluent+). For example:
- Polish Proficiency Test on TransparentLanguage.com: four sections (grammar I, grammar II, vocabulary, reading comprehension) with 10-15 questions each. For a CEFR/CECR level, the test would also need to cover writing skills, listening comprehension and oral skills.
- Language Testing International (LTI) appear to offer online tests: "Profluent+" (see also ProFluentplus.com). Although they list Polish on their Find a Test page, they don't list it on their Profluent+ page. ProFluent+ tests are rated by raters certified by the American Council on the Teaching of Foreign Languages (ACTFL) and they are online (see the FAQ). The outcome is a "ACTFL Credential of Speaking Proficiency" (see the FAQ). Apparently, these tests don't cover all skills and don't result in a CEFR level.
- polishonlinenow.com offers an accredited proficiency test. However, in spite of the organisation's name, I can't find information about taking the test online: you need to take the test in Warsaw, Gdańsk or Łódź.
- Varia courses has online placement tests (one for A1-A2, one for B1-B2) for its courses in Kraków, but placement tests don't result in official certificates.
- World Languages / Języki Świata offers an online test that is meant as a placement test, so you won't get an official certificate.
- Cactus has a test for Polish that you can do "in less than 10 minutes". Again, this test is "intended as a guide only" and won't get you a certificate.
- LanguageTrainers.com has level tests for 18 languages, but not for Polish. (I did the test for Chinese; it had 70 multiple-choice questions testing vocabulary and grammar and did not go beyond an intermediate level.)
- Tests-de-langues.com (in French) has a set of tests for Polish. Doing the tests requires registration.
- e-Testing.fr (also in French) has a test for Polish level A2. Although CEFR is an official framework, I am not sure that the certificates from e-Testing.fr can be considered as "official". I have not found how their online tests work.