[tests] Remove C++ style comments

It makes the IBM XL C Compiler (the 'native' non-free compiler
on the AIX 5.3 and 6.1 platform) stop compiling with syntax error.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=581300



Signed-off-by: Javier Jardón <jjardon@gnome.org>