* Marking database tests as slow
* Marking url command tests as slow
* Marking every test that uses database as slow
* Marking tests that import files as slow
* Marking gpg tests as slow
* Marking all versions and one list tests as slow
* Added more markers to unit tests + cli option to skip slow tests
Following a discussion with Axel, the generic 'slowtest' marker has been
split into 'db', 'network' and 'maybeslow'. A brief description of the
meaning of each marker has been added to pytest.ini.
A command line option to run only fast tests has been added to
'spack test'
* Don't use classes to group tests together
Reverted grouping tests under a class, as required in the review
* Minor style changes
