Merge topic 'set-ENV-empty'

aa5711490f set: Explicitly unset empty environment variables on Windows
723a83d8cd set: Factor out helper to set environment variables
20761cf349 set: Add test cases for setting ENV{VAR} to empty
a878d1c490 Tests: Unset the PATH environment variable more explicitly where needed

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !11290