CTest: Add error exception for sphinx-build WARNING messages

Sphinx-build warnings look like this:

    ../path/to/file.rst:105: WARNING: This is not an error.

CTest detects these as errors by this regex:

    ([^ :]+):([0-9]+): ([^ \\t])

Add an exception as we already have for lower-case `: warning`.

Source/CTest/cmCTestBuildHandler.cxx

@@ -83,6 +83,7 @@ static const char* cmCTestErrorExceptions[] = {
   "instantiated from ",
   "candidates are:",
   ": warning",
+  ": WARNING",
   ": \\(Warning\\)",
   ": note",
   "Note:",