scripts/functions: properly catch failure in CT_Test* helpers

So we get caught by the trap-handler and
have a chance to run the debug-shell.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Johannes Stezenbach <js@sig21.net>

 # Compute s390-specific values

 CT_DoArchTupleValues() {

     # That's the only thing to override

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH="s390x"

     fi

 }