s390x: intercept: make sure all output lines are unique

The intercept test has the same output line twice for two different
testcases.

Fix this by adding report_prefix_push() as appropriate.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/r/20220721133002.142897-3-imbrenda@linux.ibm.com
Message-Id: <20220721133002.142897-3-imbrenda@linux.ibm.com>