Print escaped command args

[toslunar]

I found

[DEBUG] failed to wait a command: python3.5 -m pytest -m not slow and (gpu or cudnn or cuda) /chainer/tests/chainer_tests/functions_tests/math_tests/test_average.py: signal: killed

I expect spaces are escaped or

python3.5 -m pytest -m 'not slow and (gpu or cudnn or cuda)' /chainer/tests/chainer_tests/functions_tests/math_tests/test_average.py

xpytest/pkg/pytest/execute.go

Lines 141 to 142 in e5b8314

	fmt.Fprintf(os.Stderr, "[DEBUG] failed to wait a command: %s: %s\n",
	strings.Join(args, " "), err)