The system variable spider_disable_group_by_handler, if on, will
disable the spider group by handler (gbh), and such disablement serves
as workaround to bugs caused by gbh, labelled with spider-gbh on jira,
including MDEV-26247, MDEV-28998, MDEV-29163, MDEV-30392, MDEV-31645.
Tests for these tickets are added accordingly with the workaround in
place.