HPCC-11332 Recategorize joinmulti test as a stress test

As it takes ~6 minutes to run

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5799 from richardkchapman/joinmulti-stress

HPCC-11332 Recategorize joinmulti test as a stress test

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>