Interface Kit tests: add test for BTextControl.

* Only tests very basic functionality, and size of the class, which must
not change for BeOS compatibility.
* Also add "manual" test for disabled and

Interface Kit tests: add test for BTextControl.

* Only tests very basic functionality, and size of the class, which must
not change for BeOS compatibility.
* Also add "manual" test for disabled and invalid controls (just check
it looks right)

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