cros_test: Don't raise a RunCommandError when tast tests fail.

Just a small QoL improvement. Currently when a tast test ran via
cros_test fails, the stderr prints:

cros_run_test: Unhandled exception:
Traceback (most recent call last):
  File "/b/s/w/ir/third_party/chromite/bin/cros_run_test", line 181, in <module>
    DoMain()
  ...
  File "/b/s/w/ir/third_party/chromite/lib/cros_build_lib.py", line 924, in run
    raise RunCommandError(msg, cmd_result)
chromite.lib.cros_build_lib.RunCommandError

But that's not really necessary (and mostly useless). So this change
prevents the exception from being raised, and instead lets cros_test
use its own exit code handling, which will still ensure that the
overall exit code is non-zero when tast fails.

BUG=none
TEST=`cros_run_test --tast` a failing test, rc is still 1

Change-Id: I2b56125766a83029c51b2425de1873722a12fdec
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2559150
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Tested-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
diff --git a/lib/cros_test.py b/lib/cros_test.py
index b06f918..1c13328 100644
--- a/lib/cros_test.py
+++ b/lib/cros_test.py
@@ -376,6 +376,8 @@
         cmd,
         dryrun=self.dryrun,
         extra_env=extra_env,
+        # Don't raise an exception if the command fails.
+        check=False,
         enter_chroot=need_chroot and not cros_build_lib.IsInsideChroot())
 
   def _RunTests(self):