Add a test of code map fallback behavior

[biobootloader]

Once code maps are merged (#66), write a test (not a benchmark, it should run on GH actions) that checks that calling CodeMap.get_message with different token_limits correctly transitions between a full map (with signatures), a map without signatures, and just a file map.

Reason for test: this is an important feature that seems like it be easy for us to not immediately notice if we broke it on accident

[granawkins]

This was added in #115 by this test.