Reset env if needed on every test case

[Alberdi]

In nproxy we have some tests that need to set some particular env values. To ensure that the values set in one test don't carry over to the others, I had added some code to reset it on each test case, so it doesn't matter what the previous test did to the env or if it failed before setting up the previous values. Maybe we should add some helper to nct_util to accomplish this? Not sure how common this scenario is.

Note that in nproxy is a bit more complicated because the env is saved per group and not per suite.