s390x: intercept: fence one test when using TCG

Qemu commit f8333de2793 ("target/s390x/tcg: SPX: check validity of new prefix")
fixes a TCG bug discovered with a new testcase in the intercept test.

The gitlab pipeline for the KVM unit tests uses TCG and it will keep
failing every time as long as the pipeline uses a version of Qemu
without the aforementioned patch.

Fence the specific testcase for now. Once the pipeline is fixed, this
patch can safely be reverted.

This patch is meant to go on top this already queued patch from Janis:
"s390x/intercept: Test invalid prefix argument to SET PREFIX"
https://lore.kernel.org/all/20220627152412.2243255-1-scgl@linux.ibm.com/



Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20220721133002.142897-2-imbrenda@linux.ibm.com
Message-Id: <20220721133002.142897-2-imbrenda@linux.ibm.com>