[autotest] add a --pretend flag to test_that

This CL adds a --pretend flag to test_that which causes it to print out
the autoserv commands that it would have run, rather than actually
running them.

BUG=None
TEST=Manually tested that --pretend flag works as expected. Unit tests
pass.

Change-Id: I492c67ee31319ca4c7c157f182b4108a213f047c
Reviewed-on: https://gerrit.chromium.org/gerrit/62646
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
2 files changed