Fix pydot tests when testing backends

[eriknw]

This at least partially addresses the errors in #7354. In particular, this fixes the pydot errors.

I don't know how to fix the agraph segfaults you see in #7354 @rossbar. You can work around them by removing pygraphviz from your environment.

Also, I don't think pytest-xdist has anything to do with these errors, but I like the idea of running the dispatch tests with pytest-xdist if it makes them faster.

[rossbar]

Also, I don't think pytest-xdist has anything to do with these errors

I suspect it does, at least indirectly - I always run the test suite with xdist on all my machines and have never hit these problems with the un-dispatched tests. It's only when I add the NETWORKX_TEST_BACKEND=nx-loopback to the test invocation that I see the failures reported in #7354

[rossbar]

This LGTM, thanks @eriknw !