Rename cxx-test.C to cxx-test.cpp

Meson checks file extension case-insentively, and compiles ".C" as C
source instead of C++.

tests/cxx-test.cpp

@@ -0,0 +1,10 @@
+#include <glib.h>
+#include <gmodule.h>
+#include <glib-object.h>
+#include <gio/gio.h>
+
+int
+main ()
+{
+  return 0;
+}