Only increment unit testing test number in description-creation function
Don't use "use strict" in shell unit testing harness

Our way of getting the current applicationDomain relies on arguments.callee working, and the SpiderMonkey shell's `load` function applies "use strict" to loaded code, too.
Add assertThrowsInstanceOf function to unit testing "framework"

Not used because the AVM2's `throwError` currently doesn't work without AS3 code on stack.
Fix bugs in Graphics#beginGradientFill

Turns out adding unit tests for methods really is useful.