fs_mgr: overlay: test is alarming on legacy eng builds.

On taimen-eng build, the test reports:

  [  WARNING ] user fastboot missing required to invalidate, ignoring a failure
  ERROR: expected "cat: /vendor/hello: No such file or directory"
         got "Hello World! Fri Mar 22 08:56:32 PDT 2019"
  [  FAILED  ] vendor content after flash vendor

Which is a result of a corner case problem on devices that need to
use overlayfs to support adb remount, but do not have fastbootd as
required by Dynamic Android Partitions (DAP).  These legacy non-DAP
devices we consider this a Known Issue.

The message however is too alarming when reporting this Known Issue
that the test has notified you it accepts.  Use standard notification
output format, and change the result to a series of WARNING instead.

The output would then look like:

  [  WARNING ] user fastboot missing required to invalidate, ignoring a failure
  [  WARNING ] expected "cat: /vendor/hello: No such file or directory"
  [  WARNING ] got "Hello World! Fri Mar 22 08:56:32 PDT 2019"
  [  WARNING ] vendor content after flash vendor

Which still tells us there is an issue, but does not cause a state
of extreme alarm when looking at the test output.

Test: adb-remount-test.sh
Bug: 120448575
Bug: 128876541
Bug: 123079041
Change-Id: I1d8d60f23f7670ade7eae1be629264f7507e0cfd