Add seek tests

[vweevers]

• Import tests from leveldown/test/iterator-test.js (in next branch)
  • iterator#seek throws if target is empty The empty-check is currently performed by leveldown
  • iterator is seekable
    • rename to iterator#seek() to string target
  • iterator is seekable with buffer
    • rename to iterator#seek() to buffer target
    • skip if testCommon.bufferKeys is false
  • reverse seek in the middle
    • rename to iterator#seek() on reverse iterator
  • iterator invalid seek
    • rename to iterator#seek() to out of range target
  • reverse seek from invalid range
    • rename to iterator#seek() on reverse iterator to out of range target
  • iterator optimized for seek (hooks into iterator.cache which is a leveldown detail)
  • iterator seek before next has completed
    • rename to iterator#seek() throws if next() has not completed
  • close db with open iterator (unrelated)
  • iterator seek after end
    • rename to iterator#seek() throws after end()
  • iterator seek respects range
    • rename to iterator#seek() respects range
• Add additional tests
  • target is serialized (done)
• 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List is outdated, should update after Level/leveldown#506.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.