Plugin fixtures: improve for pytest, add computer fixture to PluginTestCase

[DropD]

• the fixture context manager now always returns the same FixtureManager instance (more would be useless, because it could not reload the dbenv anyway)
• .travis-data/test_plugin_testcase.py now contains an example for a Computer fixture

Motivation for the second item: aiidateam/aiida-plugin-template#6

@ltalirz: The only significant thing i can see i am doing different is that i am ensuring that the workdir of the Computer is an existing directory.

[ltalirz]

Thanks!
I now ran the new test on my machine and it worked as well - I wonder what the problem with the plugin template was... I'll have a look!