Test-cases introduced in PYTHON-1476 are currently very fragmented, i.e. tests for the custom type en/decoding functionality currently live in multiple files, and often in multiple test classes within each file. Furthermore, each test class defines a slightly different set of custom types, and their associated codecs. This has resulted in significant code bloat which can be mitigated, in large part, by refactoring these tests such that they live in a single test file, and use a common set of custom types and encoder/decoder classes.