Add a hack to prevent the test suite from aborting after Grid test case.

Add a hack to prevent the test suite from aborting after Grid test case.

There is a problem with the native header control still having the mouse
capture when the grid is destroyed in wxGTK and this resulted in asserts when
destroying the grid, nested asserts due to an exception being thrown from a
dtor and finally abort that prevented the rest of the tests from running.

As I failed to find the real reason for this problem, just forcefully ensure
that we don't have the mouse capture when destroying the grid. This at least
allows the complete test suite to run.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71319 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775