Use "C" locale for locale-dependent part of TextCtrlTestCase.

Use "C" locale for locale-dependent part of TextCtrlTestCase.

As we expect to get decimal points in the text control when we stream floating
point numbers into it, we must do it in a locale which uses decimal point,
e.g. "C" one. Otherwise the test failed when ran in e.g. French locale.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65390 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775