test: introduce a new flag to display env variable line per line
It's easier to conditionalize some of the environment variables per Mercurial
version once there is only one value per line.
Differential Revision: https://phab.mercurial-scm.org/D5453
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed