travis: MDEV-13002 fixed - don't allow failures in MYSQL_TEST_SUITES=plugins
travis: MDEV-13002 fixed - don't allow failures in MYSQL_TEST_SUITES=plugins

Revert "travis: allowed_failures MYSQL_TEST_SUITES=plugins (MDEV-13002)"

This reverts commit c4cbc7a8807250d2d5b5938b218fd9c423a0b30a.

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2