The addition of support for open flags, within fs_open, requires
addition of new test cases.
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
Common procedure, to use with fs back-ends, have been added for testing
various fs_open flags combinations.
Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>