Add a test asserting that default fstab has /data entry

When aosp/1149619 is merged, userspace reboot will start to depend on
the fact that default fstab has an entry for /data, hence this test
to make sure that this is the case.

Bug: 135984674
Test: atest CtsFsMgrTestCases
Change-Id: I7a3c181a6b27a7b4dfc344ff04be712f3fe3376e