devinstall_test: Mount root fs r/w

BUG=chromium:950180
TEST=../platform/crostestutils/devmode-test/devinstall_test.py --verbose betty ../build/images/betty/latest/chromiumos_test_image.bin

Change-Id: Id475be7e8f5d811f70a54a3217f488e2312acde3
Reviewed-on: https://chromium-review.googlesource.com/1575012
Commit-Ready: Achuith Bhandarkar <achuith@chromium.org>
Tested-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
1 file changed