tests: fix EnvSetterTestCase when used with other tests
tests: fix EnvSetterTestCase when used with other tests

Of course, messing up with os.environ breaks
other tests ...

aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild