adb-remount-test: Refactor remount -R & disable-verity test

Since remount -R and disable-verity -R have similar expectations, group
them together and reuse each other's test code.
Remove the redundant "remount -R" test at end of file.
Total test runtime reduced by one reboot.

Bug: 243116800
Test: adb-remount-test
Change-Id: I510a9de39f94b73450df9abf82a55496df96bea1