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.

tests/mbconv/mbconvtest.cpp

@@ -1096,15 +1096,16 @@ void MBConvTestCase::TestEncoder(
     memcpy( inputCopy.data(), wideBuffer, (wideChars*sizeof(wchar_t)) );
     inputCopy.data()[wideChars] = 0;
 
-    // calculate the output size
+    // calculate the output size: notice that it can be greater than the real
+    // size as the converter is allowed to estimate the maximal size needed
+    // instead of computing it precisely
     size_t outputWritten = converter.WC2MB
         (
         0,
         (const wchar_t*)inputCopy.data(),
         0
         );
-    // make sure the correct output length was calculated
-    CPPUNIT_ASSERT_EQUAL( multiBytes, outputWritten );
+    CPPUNIT_ASSERT( outputWritten >= multiBytes );
 
     // convert the string
     size_t guardBytes = 8; // to make sure we're not overrunning the output buffer