Receive xrandr & dpms exts correctly in tests

This makes tests pass that were failing with null reference errors.

`XClient.require` was passing the callback function to `requireExt`,
which called it with parameters `null, ext` on success. The callback
itself seemed to assume it will get only one parameter, which is either
the extension on an error, hence why it tried to call methods on a
`null`.
Correspondingly rename xtest callback param

The first parameter is the error param after all.

The test logic is unchanged.
Assign to local vars instead of globals

These were being assigned to globals for some reason.

The tests return the same results when they're local, so I'll assume
this was a typo.
Check for errors directly instead of with isError
Merge pull request #72 from anko/fix-ext-tests

Fix null-reference errors from extension-tests