Use single quotes in wxMarkupText unit test to work around VC6 bug.

Use single quotes in wxMarkupText unit test to work around VC6 bug.

VC6 seems to have a strange bug with escaped quotes used in strings passed to
macros and failed to compile the original code with nonsensical error messages.

Use single quotes to try to work around this issue. They are also somewhat
more readable as they don't need to be escaped inside C strings.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67077 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775