[skip-ci] brief is for file, not for namespace

In https://root.cern.ch/doc/master/namespaces.html the description of the ROOT namespace was picked form this file brief.
Try to fix it this way.

core/testsupport/src/TestSupport.cxx

@@ -1,3 +1,4 @@
+/// \file
 /// \brief This file contains a specialised ROOT message handler to test for diagnostic in unit tests.
 ///
 /// \author Stephan Hageboeck <[email protected]>