Reduce repetition for writing tests

[sunyu0410]

Here are some ideas to reduce repetition

1. Use fixtures to set up the input data for all test. Use conftest.py to share that across multiple files
2. Use test classes to similar tests

[mjt320]

Sounds sensible to me @sunyu0410
I don't have experience with fixtures or test classes - it will probably all make sense once there's an example to look at