Enable htools unittests and coverage reports

Since we have two sets of coverage data, we make the naming
consistent:

- doc/py-coverage for Python coverage
- doc/hs-coverage for Haskell coverage

We also need to exclude some more dirs from the list of checked
directories.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>