Add tests for concurrent delete/iteration

[nolanlawson]

These tests sussed out bugs in localstorage-down
(No9/localstorage-down#61) and memdown
(Level/memdown#20), so they may be useful to add
to abstract-leveldown.

[nolanlawson]

It might be a good idea to do an audit of different *down libraries, though, and see if they're already passing or failing the tests. I believe the second test fails in MemDOWN for instance, unless I'm just not running the tests correctly:

# test batch() with missing `value`

/Users/nolan/workspace/memdown/node_modules/tape/index.js:75
        throw err
              ^
TypeError: Object #<MemDOWN> has no method '_checkKeyValue'
    at MemDOWN._batch (/Users/nolan/workspace/memdown/memdown.js:120:18)
    at MemDOWN.AbstractLevelDOWN.batch (/Users/nolan/workspace/abstract-leveldown/abstract-leveldown.js:162:17)
    at Test.<anonymous> (/Users/nolan/workspace/abstract-leveldown/abstract/batch-test.js:20:8)
    at Test.bound [as _cb] (/Users/nolan/workspace/memdown/node_modules/tape/lib/test.js:59:32)
    at Test.run (/Users/nolan/workspace/memdown/node_modules/tape/lib/test.js:72:10)
    at Test.bound [as run] (/Users/nolan/workspace/memdown/node_modules/tape/lib/test.js:59:32)
    at Object.next [as _onImmediate] (/Users/nolan/workspace/memdown/node_modules/tape/lib/results.js:66:15)

[nolanlawson]

/cc @calvinmetcalf who wrote the first test

[calvinmetcalf]

this should be 'b' if they have a snapshot

[nolanlawson]

Small fix pointed out by @calvinmetcalf - if you delete 'b' after iterating past 'a', then the next iterated element will be 'b' on backends that support snapshots, but 'c' on backends that don't. In any case, localstorage-down was returning undefined, so we can just test for that.

[ralphtheninja]

I'm rebasing this branch.

[vweevers]

Superseded by #239.