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.

module_build_service/builder/utils.py

@@ -44,12 +44,11 @@ def execute_cmd(args, output=None, cwd=None):
 
     log.info("Executing the command \"%s\"%s" % (" ".join(args), out_log_msg))
     proc = subprocess.Popen(args, stdout=output, stderr=output, cwd=cwd)
-    out, err = proc.communicate()
+    proc.wait()
 
     if proc.returncode != 0:
         err_msg = "Command '%s' returned non-zero value %d%s" % (args, proc.returncode, out_log_msg)
         raise RuntimeError(err_msg)
-    return out, err
 
 
 def get_koji_config(mbs_config):