Add test coverage

[schwehr]

A large portion of geemap does not have test coverage. Looking for help from anyone. Contributions from a single test function to covering an entire file are welcome.

General things about tests:

• Use unittest
• Run the presumbit for formatting
• Tests should not talk to the Earth Engine service or anywhere on the internet
• Generally try to follow https://google.github.io/styleguide/pyguide.html

If you get stuck, please create a new issue for your test that llinks to what you have so far and the what has you stuck.