Fix CtsFsMgrTestCases

As ReadFstabFromFile() may append / remove / modify the fstab read from
the file, we cannot make assumptions about the number of fstab entries.
We can however test that the returned fstab contains at least the
entries we expect.

Fixes: 185826755
Test: atest CtsFsMgrTestCases on GSI & DSU
Change-Id: I539e7eed3f7ae14db7e9983bed7f68754c9fff39