Coverage report path should be the parent of test_runner_base_path

For mobly, the test_runner_base_path is the directory which contains the
test class name (<Timestamp>/<TestClass>) so we only generated the
coverage report for each class, not the entire test run. Merge
everything and store it in the root of this test suite (<Timestamp>).

Test: cert/run
Bug: 199439539
Tag: #refactor
Change-Id: If760723150b4ae6a6a91dd8ed3100973b394829f