fs: tests: Use 'fs' instead of 'filesystem' as directory name

The convention everywhere else in the tree is to use 'fs' for file
system related code (e.g. subsys/fs and include/fs) so follow this
also for the tests.

Change-Id: I32b9918cbba95e7b219c2d9f3e3a079e664390ab
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>