Added simple tests for database and cache locations.
Moved test PATH setup to new file-level function which should slightly
speed up testing and makes more sense conceptually to set the bemoji
executable to be in the path once for all tests.
Added a quick way to see if directories have been set up correctly. This
also eases testing for future ways of setting directories through
environment variables or options.
Now that the project is finding a little adoption it is really high time
to bring some tests into the code to ensure everything works as it
should.
This first version will only provide a simple test harness from which
tests can slowly be written for other parts of the program.
