network_WiFi_FastReconnectInDarkResume: fix test logic

The test should measure the difference in dark resume count instead of
looking directly at the dark resume number.

BUG=chrome-os-partner:43894
TEST=`test_that --fast -b oak dut network_WiFi_FastReconnectInDarkResume
      --iterations 3`

Change-Id: If7433be6affa54e0c7aea358f6dfe6b57d2dffe2
Reviewed-on: https://chromium-review.googlesource.com/319957
Commit-Ready: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>
Reviewed-by: Samuel Tan <samueltan@chromium.org>
1 file changed