use ES6 classes in tests to please TypeScript

Otherwise TypeScript doesn't see that Test inherits
AbstractLevelDOWN, if defined like this:

```
function Test (location) {
  AbstractLevelDOWN.call(this, location)
}

util.inherits(Test, AbstractLevelDOWN)
```

And will complain about AbstractLevelDOWN methods being undefined on
Test.

abstract/put-get-del-test.js

@@ -1,3 +1,5 @@
+'use strict'
+
 var db
 const verifyNotFoundError = require('./util').verifyNotFoundError
 const testBuffer = Buffer.from('testbuffer')