Fix fs_mgr_unit_test

The copy of /data/* doesn't work now, so put the fstab content
into the unittest source instead.

Also replacing ReadFstabFromFd with ReadFstabFromFile, to prevent
race.

Finally, two test cases are temporarily disabled due to read
fstab file error (root cause is still unknown).

Bug: 124837435
Test: atest 124837435
Change-Id: Ib6a3d931a48bffd8be23bda23fa4492babafd166