adb-remount-test: Discover fstab pathname more intelligently

Pick exactly one fstab file whose pathname suffix matches one of the
fstab suffix properties.
This helps on CF who ships redundant copies of fstab.

Bug: 243116800
Test: adb-remount-test
Change-Id: I4d38859014161e14dba1f7e19dbce44a2621d0f1