Replace anonymous namespace with a named one in the test helpers.

This should avoid clang warnings about the function in anonymous namespace
being unused.

This is the backport of r76928 from master.

tests/asserthelper.cpp

@@ -14,7 +14,7 @@
 
 #include "asserthelper.h"
 
-namespace
+namespace wxTestPrivate
 {
     std::ostream& operator<<(std::ostream& os, const ColourChannel& cc)
     {
@@ -23,10 +23,12 @@ namespace
         os << static_cast<int>(cc.m_value);
         return os;
     }
-} // anonymous namespace
+}
 
 std::ostream& operator<<(std::ostream& os, const wxColour& c)
 {
+    using wxTestPrivate::ColourChannel;
+
     os << std::hex << std::noshowbase
        << "("
        << ColourChannel(c.Red()) << ", "