tests: load showstack in test-pull-http.t so network hangs are easier to find This also gives us some minimal "it loads" coverage on showstack, which I rather like. showstack doesn't work on Windows per mbarbison, so it's disabled there. I added this in service of debugging a hang introduced on Python 3 by revision a88d68dc3ee8. I'm still not sure what the problem there is, but this at least gives us a little bit of a chance to figure out what's going on. Differential Revision: https://phab.mercurial-scm.org/D3247