adb-remount-test: Refactor check_eq & check_ne

Just use bash [[ for regex compare, which result in shorter code and
more robust.
Simplify the messaging pipeline:
  If success, don't print anything and return 0. Let caller decide what
  to log.
  If failure and --warning, log error message and return 1.
  If failure and ! --warning, die with error message.

Bug: 243116800
Test: adb-remount-test
Change-Id: Ie5426ff3fa57395aa6b4fe71c9bf96bd8e9afc35