Tests: Allow post_action to determine exit status

[kapoorlakshya]

Add new attr post_action_determines_exit_code to let the test runner post action determine the exit status. Our use case is allow soft failing of certain test targets.

ios_xctestrun_runner(
    name = "ios_sim_runner_with_soft_fail",
    device_type = "iPhone Xs",
    post_action = ":post_action_soft_fail",
    post_action_determines_exit_code = True,
)

[luispadron]

I like this but could we make this an actual attr of the test runner in addition to an env var? It's more discoverable that way

[kapoorlakshya]

I like this but could we make this an actual attr of the test runner in addition to an env var? It's more discoverable that way

@brentleyjones and I discussed this earlier and the reason for this approach is to allow toggling this from a .bazelrc. Brentley, thoughts?

[brentleyjones]

For us I guess it doesn't matter, since we have a macro and can apply it to every instance. The environment variable has the nice effect of (1) easily applying it globally if you want (2) automatically passed into post_action where we can check it if we want to.