Add a test for bump_geemap_version.py

I have a draft test file for the bump_geemap_version script and I need to put it into the repo. I'm not quite sure where to put it and it likely has some minor things to fix as I created it in google3 with bazel.

https://gist.github.com/schwehr/dc95fcc5d029a602220ddce2145f6b1b