Backend tests should not message the widget

[tinchodias]

Tests in Spec2-Backend-Tests should query the state of the widget through adapter's API. Most tests are fine, but some of them make assumptions on the widget's API, and send messages to the widget that only Morphic backend understand.

I collected this list:

• 8 tests in SpGridLayoutAdapterTest send self presenter adapter widget submorphs, assuming backend is Morphic
• SpCheckboxAdapterTest>>#testChangingLabelAffectTheWidget
• SpLabelAdapterTest>>#testBackgroundColorChangesColor
• SpNotebookAdapterTest>>#testSelectingPageShouldAnnounceChangeEvent (sends self adapter widget tabSelectorMorph)
• SpRadioButtonAdapterTest>>#testChangingLabelAffectTheWidget (sends self widget label)
• 3 tests in SpTextAdapterTest send presenter adapter widget textArea editingState undoManager

I extracted those candidates by exploring the senders of widget in the Spec2-Backend-Tests package: