Tests: Increase variation in generated tree shapes

[sjakobi]

Addresses #438.

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TODO:

• Use the same Key type for HashSet and Strictness tests
• Try producing Full nodes?!
• Check that any tests that use specific colliding keys still do the right thing.
• Improve shrinking for SmallSum (shrinkBoundedEnum?!  nick8325/quickcheck#343)

[sjakobi]

[ ] Try producing Full nodes?!

I think this might be a bit tricky. I don't think we can achieve this by tweaking the Key generator. I think we'd have to tweak the [(Key,X)] inputs for the property tests, which are currently using the default Arbitrary [] instance.

[sjakobi]

[ ] Try producing Full nodes?!

I think this might be a bit tricky. I don't think we can achieve this by tweaking the Key generator. I think we'd have to tweak the [(Key,X)] inputs for the property tests, which are currently using the default Arbitrary [] instance.

I'll leave this for future work. Ideally we can solve this by removing the Full constructor (#399).

[sjakobi]

[ ] Try producing Full nodes?!

I think this might be a bit tricky. I don't think we can achieve this by tweaking the Key generator. I think we'd have to tweak the [(Key,X)] inputs for the property tests, which are currently using the default Arbitrary [] instance.

I'll leave this for future work. Ideally we can solve this by removing the Full constructor (#399).

Tracked in #445.