Cert: generate coverage report in teardown_class

* Generate coverage report in teardown class to same time
* Merge coverage data after each test case
* But only generate coverage report after running all tests in a class
* It takes about 1 second to merge coverage data and 2 seconds to
  generate test reports. Hence this CL would roughly save 2 seconds for
  each test case
* This CL assumes that all test cases in the same test class use the
  same underlying stack binary
* An assert is added to make sure this is true

Bug: 194826691
Test: gd/cert/run
Tag: #gd-refactor
Change-Id: I108c580ea8755fa8a161bef1ddda90a49bc7a430