cleanup tests

[phiryll]

Still much more to do here. Not testing by key length is next.

• Clean up one-off unit tests.
• Move traverser-specific test functions to traversers_test.go.
• Inline some helper functions that are only used once.
• Move randomization functions to bench_test.go.
• Add helper function for testing an early yield().
• Compare iterators to avoid building slices.
• Structure bench and test names as config first, impl second.
• Have reference cache sorted keys.
• No need to clone the store in BenchmarkGet.
• Remove some benchmarks for ChildBounds.
• Remove some unneeded unit tests.