E3-1751: Fix make test to use cellbuild's path

As make test creates a cellbuild, the tests should look for modules in the cell directory.

This removes the need for activating an environment as part of any custom test targets.

This would allow module devs to go from

# configure/module/RULES_MODULE

module_tests:
    source /some/env && pytest

# tests/test_something.py

with IOC("--cell-path", os.environ["TEMP_CELL_PATH"]):
  assert things

to

# configure/module/RULES_MODULE

module_tests:
    pytest

# tests/test_something.py

with IOC():
  assert things