Disable tests for UTF-encoded files in FileTestCase in ANSI build.

Disable tests for UTF-encoded files in FileTestCase in ANSI build.

These tests didn't work correctly in ANSI build because the conversion
parameter of wxFile::Write() isn't used there, the contents of an ANSI
wxString is always written to the file as is -- hence reading it back using
UTF-16 or UTF-32 conversion fails.

The test would need to be totally rewritten for ANSI build of wx and it
wouldn't test wxFile but rather conversion functions already tested elsewhere
so just disable it instead.

This fixes a crash (due to passing NULL pointer to memcmp()) which prevented
the test suite from running to completion in ANSI build.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65834 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775