test: detect if we're running inside gdb and disable forking

The Check test framework forks by default which is annoying when running gdb.
Try to detect whether we're inside gdb by ptracing ourselves. If that works,
we're not inside a debugger. If it doesn't, then assume we're inside a
debugger and set CK_FORK to "no".

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>