make testCommon default to abstract-leveldown/testCommon.js

[ralphtheninja]

testCommon-parameter is required from leveldown and passed in to most tests of abstract-leveldown, it should default to local testCommon.js if not defined

Examples:

https://github.com/Level/leveldown/blob/990e22fa1cf07fdc011d594f7ce9e025f458d7ee/test/approximate-size-test.js#L2
https://github.com/Level/leveldown/blob/990e22fa1cf07fdc011d594f7ce9e025f458d7ee/test/batch-test.js#L2
https://github.com/Level/leveldown/blob/990e22fa1cf07fdc011d594f7ce9e025f458d7ee/test/chained-batch-test.js#L2
https://github.com/Level/leveldown/blob/990e22fa1cf07fdc011d594f7ce9e025f458d7ee/test/del-test.js#L2

etc