check(first, ...)

Run a command, and verify the results:
	check(torun, ret, stdout, stderr, stdin)
		torun	list of strings, {"program", "arg1", "arg2" ,...}
		ret	expected return code
		std{out,err}
			false	ignore output
			true	save the output in a file named stdout or stderr
			string	the string must match the output
			{string} the named file must match the output
			nil (not given)	the output must be empty
		stdin
			true	use existing "stdin" file
			nil, false	empty input
			string	contents of string are used as input
			{string} contents of named file are used as input
	check(bool)
		Throw an error if the argument is false.
	check(number)
		Check that the argument is zero.
If the command fails, abort the test.