Cert: Add --repeat option to gd/cert/run

* This option allows a test to run multiple times within one setup
  so that test owners can test the robustness of their tests

Test: source gd/cert/run --host --test_filter=CertSelfTest --repeat=5
Bug: 151989046
Change-Id: I8e057772d6a296d3c20ced0d68e3afcf0052a682