Disable test unit.pcre_test on s390x that was failing in stretch-security

Upstream developer confirmed the test is unimportant and it is OK
to allow it to fail.

https://jira.mariadb.org/browse/MDEV-17865

debian/unstable-tests.s390x

@@ -15,3 +15,4 @@ rpl.rpl_row_sp001 : MDEV-9329
 rpl.rpl_insert : MDEV-9329
 rpl.rpl_insert_delayed : MDEV-9329
 main.delayed : https://jira.mariadb.org/browse/MDEV-9679
+unit.pcre_test : MDEV-17865