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>

run_tests

@@ -50,7 +50,7 @@ def main(argv):
   os.environ['PYTHONPATH'] = pythonpath
 
   pytest = find_pytest()
-  return subprocess.run([pytest] + argv, check=True)
+  return subprocess.run([pytest] + argv, check=False).returncode
 
 
 if __name__ == '__main__':