Fix FormatConverterTestCase for MinGW and Cygwin

Standard/Unix format strings are intentionally used when using these
compilers/environments (see d62c535d4c for
the former), so don't check for Windows/MSVC format strings in the test.

tests/formatconverter/formatconvertertest.cpp

@@ -311,7 +311,9 @@ void FormatConverterTestCase::check(const wxString& input,
 #if wxUSE_UNICODE && !wxUSE_UTF8_LOCALE_ONLY
     result = (const wchar_t*)wxFormatString(input);
 
-#ifdef __WINDOWS__
+#if defined(__WINDOWS__) && \
+    !defined(__CYGWIN__) && \
+    !defined(__MINGW32__)
     wxString expectedWchar(expectedWcharWindows);
 #else
     wxString expectedWchar(expectedWcharUnix);