Generate test coverage reports

[rbehjati]

A test coverage report can be generated using go test -cover (see https://tip.golang.org/doc/go1.2#cover). However, we cannot use it directly on the top-level repo, since we cannot run the authorization logic tests without bazel.

A test coverage report can be generated with bazel (https://docs.bazel.build/versions/main/coverage.html). But I was not able to generate html reports using genhtml on output generated by the bazel coverage --combined_report=lcov [target] command.