Add seek tests

[vweevers]

• [x] Import tests from leveldown/test/iterator-test.js (in next branch)
  • [x] iterator#seek throws if target is empty The empty-check is currently performed by leveldown
  • [x] iterator is seekable
  • [x] rename to iterator#seek() to string target
  • [x] iterator is seekable with buffer
    • [x] rename to iterator#seek() to buffer target
    • [x] skip if testCommon.bufferKeys is false
  • [x] reverse seek in the middle
  • [x] rename to iterator#seek() on reverse iterator
  • [x] iterator invalid seek
  • [x] rename to iterator#seek() to out of range target
  • [x] reverse seek from invalid range
  • [x] rename to iterator#seek() on reverse iterator to out of range target
  • [x] iterator optimized for seek (hooks into iterator.cache which is a leveldown detail)
  • [x] iterator seek before next has completed
    • [x] rename to iterator#seek() throws if next() has not completed
  • [x] close db with open iterator (unrelated)
  • [x] iterator seek after end
  • [x] rename to iterator#seek() throws after end()
  • [x] iterator seek respects range
  • [x] rename to iterator#seek() respects range
• [x] Add additional tests
  • [x] target is serialized (done)
• [x] Test that implementation doesn't define public seek, which is currently the case in leveldown and friends. They should define _seek instead. This would require Object.getPrototypeOf shenanigans, making assumptions about the implementation and its prototype chain.

[vweevers]

Added a list of tests, so we can easily see which ones can be removed from leveldown later.

[vweevers]

List is outdated, should update after https://github.com/Level/leveldown/pull/506.

[ralphtheninja]

@vweevers Nice that you're hacking a bit on level. I have been doing other stuff for a while. Will try to get back into level shortly.

[vweevers]

List updated.