Update tests concerning null-extending classes

[jugglinmike]

The latest revision of ECMA262 makes special provisions for classes
which extend the null value [1]. Update the relevant tests
accordingly.

[1] tc39/ecma262#543

@bterlson I believe this fully addresses gh-588. As far as I can tell, there
are two observable difference for that change in the spec:

• There is no requirement for the constructor to return a value (ES2016
  required a return value in this case, and I've updated the relevant test)
• this may be referenced from within the constructor (ES2016 does not allow
  such a reference, but we previously had no test for that)

SuperCall continues to be invalid for the same reason. Despite the fact that
there was no change to that behavior, I added a test for good measure.

I'm reasonably confident that there are no lingering tests for the outdated
semantics. (My heuristic is manually inspecting all files that contain the
strings "extends" and "null" but not necessarily on the same line.)

$ grep -E 'extends' test/ -r -l --null | xargs -0 grep null -l

This should resolve gh-588.

[leobalter]

It's great to have this test and class-definition-null-proto-super.js

[leobalter]

just a few descriptive content to fix, the tests LGTM.

[jugglinmike]

All set!