BACKPORT: Add support in Snap.Snaplet.Test for reusing initializers.

This was a change that was originally against 0.13, but got merged after master switched to 1.0 (the same day, even!). I'm in the process of doing a proper release of a testing library that could really use these changes to be on the current stable (it speeds up test suites by 10-100x). All tests pass on 0.13 with these changes.