Support running test suite on a levelup db

[vweevers]

As well as skipping 'start' and 'end' tests (for multileveldown) (see also Level/community#86).

Doesn't change anything for existing abstract-leveldown implementations. Before releasing I do want to test it on:

• multileveldown (as a levelup instance)
• subleveldown (same)
• level

Working towards Level/community#58.

[vweevers]

Almost works on level!

var test = require('tape')
var factory = require('level')
var tempy = require('tempy')
var suite = require('abstract-leveldown/test')

suite({
  test: test,
  factory: function () {
    return factory(tempy.directory())
  },
  clear: true,
  createIfMissing: false, // TODO: fails
  errorIfExists: false, // TODO: fails
  promises: true,
  status: false,
  serialize: false,
  encodings: true
})

And it also works when you take out encoding-down:

var factory = require('level-compose')(require('leveldown'), require('levelup'))

// same as above, but with `encodings: false`
// ..

[vweevers]

The createIfMissing and errorIfExists tests can't be fixed without a semver-major change, because they do:

test('test database open createIfMissing:false', function (t) {
  // levelup db, opens itself
  var db = testCommon.factory()

  // options are ignored because it's already opening
  db.open({ createIfMissing: false }, function () { /* .. */ })
})

The easiest way forward is probably to move such options to the (abstract)-leveldown constructor.