HPCC-12090 Add a separate environment.xml for the regrssion suite

Includes a 3-slave thor.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #6427 from jakesmith/hpcc-12090

HPCC-12090 Add a separate environment.xml for the regrssion suite

Reviewed-By: Attila Vamos <Attila.Vamos@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>