tests: append instead of override environment

Some tests that rely on some environment variables being passed to child
processes would fail because they reset the child processes'
environement instead of appending to it. This would break on test
environments where some custom environment variables are needed to make
node work properly.

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>