run test_that.py within an ssh-agent environment

This CL ensures that test_that.py is run in an environment with a
running ssh-agent, to which it can add identities with `ssh-add` if
necessary. Calling ssh-agent this way ensures that it will terminate
once test_that.py terminates.

CQ-DEPEND=CL:I83fe442cdd5a3ef3594cefa2a818d56c576d5969
BUG=chromium:242987, chromium:274047
TEST=see CQ-DEPENDED CL.

Change-Id: If48fb7e9e0aa1e9744a172db42f1a03d9b22aa56
Reviewed-on: https://gerrit.chromium.org/gerrit/66231
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
1 file changed