fs_mgr: adb-remount-test.sh: Port to MAC OS/X

This script did not run on a MAC, adjust so that it is usable.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 161454607
Test: script can be used to test and replicate reported problem
Change-Id: Id1c97b9cd85d150a96733b8d39e40f6a4bcc0721