Changes to Custom Set tests

[IanWhitney]

We recently rewrote the test suite for Custom Set. Since this track implements Custom Set, please take a look at the new custom_set.json file and see if your track should update its implementation or tests.

The new test suite reorders tests so that students can get to green quickly. It also reduces the number of tests so that students can focus on solving the interesting edge cases.

More details on this change are available in the pull request

[masters3d]

closing in favor of #146