This could never work without on-site invigilated exams. Otherwise you will never be able to prove that when Jack applies for a job, Jack took the test. You'll

This could never work without on-site invigilated exams. Otherwise you will never be able to prove that when Jack applies for a job, Jack took the test. You'll just get an industry of Johns taking the tests for people. This is how language certification works (think TOEFL, IELTS), and that industry still suffers from a massive amount of fraud. If you make a certificate like this too expensive, it serves as a barrier for entry for candidates. If you make it too cheap, the people administering the test will start pushing through candidates as quickly as possible without any regard for academic honesty.