elpy-test: Ask to save unsaved buffers.

Running tests but having forgotten to save the buffer can result in
very confusing results. We now query the user to save modified
buffers before running tests.
locate-dominating-file: Use the backported version in 24.2, too.

This appears not to be included in 24.2 yet, even though the
change happened before the release date. I guess it wasn't ready
for the freeze.

Fixes #31