Initialize all environment variables when running isctest

Initialize all environment variables when running isctest

Ensure all the variables are initialized when running the main function
of isctest module. This enables proper environment variables during test
script development when only conf.sh is sourced, rather than the script
being executed by the pytest runner.

(cherry picked from commit d7ace928b5e9520b0b68169abef43be3f6d50a0c)