Don't rely on wxMBConv::cWC2MB(NULL) returning the exact byte count

UTF-32 conversions use a useful optimization by avoiding the extra scan of the
input wchar_t string in their FromWChar() and cWC2MB() implementation when
they are only asked to compute the required buffer size without actually doing
the conversion. However this means that for an input string containing UTF-16
surrogates (which is possible under MSW where wchar_t is 16 bits) the actual
size of the output string can be smaller than that returned by FromWChar(NULL).

Document that this may happen and avoid relying on the exact equality in the
tests.

See #17070.

interface/wx/strconv.h

@@ -121,7 +121,7 @@ public:
             including the terminating @c NUL character(s).
 
         @return
-            The number of character written (or which would have been written
+            The number of characters written (or which would have been written
             if it were non-@NULL) to @a dst or @c wxCONV_FAILED on error.
     */
     virtual size_t ToWChar(wchar_t* dst, size_t dstLen, const char* src,
@@ -148,8 +148,13 @@ public:
             including the terminating @c NUL character.
 
         @return
-            The number of character written (or which would have been written
-            if it were non-@NULL) to @a dst or @c wxCONV_FAILED on error.
+            If @dst is non-@NULL, the number of characters actually written to
+            it. If @dst is @NULL, the returned value is at least equal to the
+            number of characters that would have been written out if it were
+            non-@NULL, but can be larger than it under the platforms using
+            UTF-16 as @c wchar_t encoding (this allows a useful optimization in
+            the implementation of this function for UTF-32). In any case,
+            @c wxCONV_FAILED is returned on conversion error.
     */
     virtual size_t FromWChar(char* dst, size_t dstLen, const wchar_t* src,
                              size_t srcLen = wxNO_LEN) const;