Add description to Test form

[disconcision]

The idea is we want to be able to attach a description to a test + display that as a hint in the exercises mode UI.

Currently, hints are tracked separately in the exercise data structure. Instead, we can add a Hazel language feature to support this.

Until we add optional arguments to Hazel, we can define a separate keyword:

Current keyword:

test e end

Hinted version:

hinted_test "Hint" for e end