Check usefulness of tests

[dhardy]

Several distribution tests merely generate a sample then discard it, e.g. test_normal. This checks that the syntax is still functional and that no internal asserts happened, but is probably not very useful.

• Remove useless tests
• Create tracker issue(s) for desired test functionality (or implement)

Also see: dhardy#72

[dhardy]

Also note that some tests are duplicated due to replacing ind_sample with sample everywhere; duplicates can be removed.

[dhardy]

Work on this isn't complete, but I don't see any value in keeping the issue open. #357 is the more useful tracker.