fs_mgr: overlayfs: test: adding diagnostic value

Add a test report df before overlay applied to make it clearer what
the device looked like before applying for remount.  Check error
code from adb disable-verity and use that to terminate test if bad.
Collect a sliced logcat for failures to help diagnose problems.
If the device does not come back after flashing, timeout and fail
when waiting.  Fix some issues that result when running script under
set -x when diagnosing the tests logistics.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: Iffbd6261703393417cb6ec3a12042d59523c348d