add push hook that runs test suite

git-hooks/README.md

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+To use these hooks, cp any of them to onionshare's `.git/hooks`.
+
+* `pre-push` runs the test suite, and will push if the tests pass.

git-hooks/pre-push

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+#!/bin/bash
+
+# Pre-push hook. If you want to test with a different version of firefox, put
+# the path in the CFX_FIREFOX environment variable.
+
+nosetests test