builder/utils/execute_cmd: remove useless return value and add a test

Since we never pass in subprocess.PIPE for stdin/stdout/stderr,
subprocess.communicate() does nothing, and the return out/err tuple
was always empty.

tests/test_builder/test_builder_utils.py

@@ -17,6 +17,22 @@ from module_build_service.scheduler.db_session import db_session
 from tests import read_staged_data, scheduler_init_data
 
 
+def test_execute_cmd(tmpdir, caplog):
+    logfile = str(tmpdir / "out.log")
+    with open(logfile, "w") as f:
+        utils.execute_cmd(["echo", "hello"], output=f)
+
+    with open(logfile) as f:
+        assert f.read() == "hello\n"
+
+    assert 'Executing the command "echo hello", output log: %s' % logfile in caplog.text
+
+
+def test_execute_cmd_fail():
+    with pytest.raises(RuntimeError):
+        utils.execute_cmd(["false"])
+
+
 @pytest.mark.parametrize("variation", ("none", "empty", "already_downloaded"))
 @patch("requests.get")
 @patch("koji.ClientSession")