Fix CBFS comparator with our_check_output()

The cbfs tests were broken by fe446ea2

They would fail with: TypeError: a bytes-like object is required, not 'str'

I wasn't initially sure if decode() should be in our_check_output(), but then I noticed we have other places where we call decode() after running our_check_output(), so I'm following that.

Additionally, our_check_output() can't accept a stdout= argument since subprocess.check_output doesn't accept this argument.