run_tests: fix exit code handling

We need to pass back an int, not a CompletedProcess object.  Switch to
check=False so we don't throw an exception on failure -- we're already
showing pytest's stderr, and will return the non-zero status.

Change-Id: Ib0d3862a09a3963f25025f39a8e34419cf2a54df
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299624


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>