Added test case for expected browser execCommand "true" return

Thank you for taking the other pull requests I sent. I discovered that (for now?) TinyMCE doesn't implement "InsertLineBreak" on its own and it ultimately passes through to the browser where it is handled with "true" return value, at least on WebKit. In case this is helpful it would supercede my comment in the previous tests that lamented the lack of such an example. I'm not sure though how it behaves for other platforms.