(PUP-3334) Ensure original env is correct in test

Prior to this commit, there was no expect in the test to make sure
that the original environment was correct before the settings were
changed. This meant that it was possible for the test not to work
as expected but still pass. Add a second expect statement to ensure
that we have the right environment to begin with.

Additionally, instead of modifying with_modulepath to use
full_modulepath, add a with_full_modulepath option. That way it
doesn't change what the other tests are testing.