update tests to use String path API for the db/container file
rename long/short key tests and variables for clarity

These tests check the response when the key is either too long or too short
so that should be clearer in the methods and variables.
catch IllegalStateException for unmounts when mounts are supposed to fail
testing whether everything works after a mount() exception

If someone calls mount() on a mounted vfs, it throws an Exception, but
everything should still continue working fine.
increase the size of some tests to test more real world conditions

These tests were writing some very small files.