Fixed GafferTest.BadNode to derive from ComputeNode.

It was totally broken but going unnoticed because its role in the test cases is to do things wrong.
Removed expectedFailure test case.

Realistically we can never make this one pass - and it doesn't expose a bug, just a feature that would be very nice.
Removed expectedFailure decoration for test which now passes.
Marked ScrolledContainerTest.testChildAccessors as an expected failure.

There's little hope as fixing this as it appears to be a problem on the Qt side. Using expectedFailure for this test required some jiggery pokery in the TestCase class, which is documented in the comments there.
Fixed bugs caused by recent merges.

The Plug::ReadOnly serialisation changes and the CompoundDataPlug::MemberPlug changes weren't compatible.