tests/test-debugindexdot.out

replace usage of os.popen() with util.popen()

To make this possible, I added a mode parameter to both implementations of
util.popen(), defaulting to 'r' (as it does in the Python stdlib).

created new head
digraph G {
	-1 -> 0
	0 -> 1
	0 -> 2
	2 -> 3
	1 -> 3
}