Fix occasional issue tests writing with htmlcov to data dir
Fix occasional issue tests writing with htmlcov to data dir

Occasionally one can run into an issue where htmlcov gets written to the current working directory (cwd) after a test was run that changes the cwd. In some cases we change the cwd in our tests to point at the `nebu/tests/data` directory. For some unknown reason the htmlcov files get written into that directory. This fix changes the gitignore to only ignore the project root `htmlcov`; thus allowing the developer to see the directory in a `git status`.

Connexions/nebuchadnezzar