Poison netifaces.interfaces() in tests

While reviewing [1] it was noted that we've probably been invoking the
real netifaces.interfaces() in some test paths. This isn't good, so
introduce a fixture that blows up any test that does that.

(NB: during development, the netifaces.interfaces()-specific poison
fixture evolved into a generic fixture that can be extended in the
future to poison anything in a similar fashion.)

[1] https://review.opendev.org/#/c/671471/

Change-Id: I1fea14d5be10bb4e884f52e0ae8be722519ddd3f

nova/test.py

@@ -285,6 +285,8 @@ class TestCase(base.BaseTestCase):
         quota.UID_QFD_POPULATED_CACHE_BY_PROJECT = set()
         quota.UID_QFD_POPULATED_CACHE_ALL = False
 
+        self.useFixture(nova_fixtures.GenericPoisonFixture())
+
     def _setup_cells(self):
         """Setup a normal cellsv2 environment.
 

nova/tests/fixtures.py

@@ -2672,3 +2672,23 @@ class CyborgFixture(fixtures.Fixture):
             'nova.accelerator.cyborg._CyborgClient.'
             'delete_arqs_for_instance',
             side_effect=self.fake_delete_arqs_for_instance)).mock
+
+
+class GenericPoisonFixture(fixtures.Fixture):
+    POISON_THESE = (
+        ('netifaces.interfaces',
+         'a test environment should not be inspecting real interfaces on the '
+         'test node'),
+    )
+
+    def setUp(self):
+        def poison_configure(method, reason):
+            def fail(*a, **k):
+                raise Exception('This test invokes %s, which is bad (%s); you '
+                                'should mock it.' % (method, reason))
+            return fail
+
+        super(GenericPoisonFixture, self).setUp()
+        for meth, why in self.POISON_THESE:
+            self.useFixture(fixtures.MonkeyPatch(
+                meth, poison_configure(meth, why)))