plugin_test.py: Coverity: Correct assertions
plugin_test.py: Coverity: Correct assertions

Re-order assertions and check to ensure fs is not None before
dereferencing it.

Signed-off-by: Tony Asleson <1ef2d381aa56d512126ddaacc29e68136e4fdf55@redhat.com>

tasleson/libstoragemgmt,cathay4t/libstoragemgmt,joehandzik/libstoragemgmt,cathay4t/libstoragemgmt,jumitche/libstoragemgmt,joehandzik/libstoragemgmt,jumitche/libstoragemgmt,tasleson/libstoragemgmt,cathay4t/libstoragemgmt,tasleson/libstoragemgmt,joehandzik/libstoragemgmt,jumitche/libstoragemgmt,jumitche/libstoragemgmt,tasleson/libstoragemgmt,libstorage/libstoragemgmt,cathay4t/libstoragemgmt,tasleson/libstoragemgmt,libstorage/libstoragemgmt,joehandzik/libstoragemgmt,cathay4t/libstoragemgmt,jumitche/libstoragemgmt,libstorage/libstoragemgmt,libstorage/libstoragemgmt,libstorage/libstoragemgmt