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 Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }