GPFS unit tests: increased coverage, uses mock

Increased unit tests coverage for the GPFS driver 100%.
All tests make usage of mock.
This helped to uncover bugs in the error path (see 1289382).

Closes-bug: 1290921

Change-Id: Ic78ac9b49c6e22231b966052f65d8ae07824379e
Merge "GPFS unit tests: increased coverage, uses mock"