Clear shared IP cache in parallel test setUp for isolation (#866)

tests/test_parallel.py's parity test compares sequential
parse_report_file(path, offline=True) results in the parent process
against results from cold worker processes. When the full suite runs
locally (GITHUB_ACTIONS unset), tests/test_init.py runs first with real
DNS lookups and warms the shared module-level
parsedmarc.IP_ADDRESS_CACHE; get_ip_address_info consults the cache
before honoring offline, so the sequential baseline returned
DNS-enriched entries (e.g. reverse_dns='smtp7.cardinal.com') while the
workers correctly returned None, failing the test. CI never sees this
because it runs offline from the start.

Clear the cache in _ParallelTestCase.setUp so baselines and workers
both start cold. Test-only change; the cache-before-offline ordering in
utils.py is intentional (a cache hit makes no network queries).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/test_parallel.py

@@ -47,7 +47,8 @@ class _CountingIterable:
 
 
 class _ParallelTestCase(unittest.TestCase):
-    """Common env setup shared by parallel.py tests: offline mode, no DNS."""
+    """Common env setup shared by parallel.py tests: offline mode, no DNS,
+    and a cold IP address cache."""
 
     def setUp(self):
         self._env_patcher = patch.dict(
@@ -55,6 +56,13 @@ class _ParallelTestCase(unittest.TestCase):
         )
         self._env_patcher.start()
         self.addCleanup(self._env_patcher.stop)
+        # Earlier test modules (e.g. tests/test_init.py) may run with DNS
+        # enabled locally and warm the shared module-level
+        # parsedmarc.IP_ADDRESS_CACHE. get_ip_address_info consults the
+        # cache before the offline check, so a warm cache would leak
+        # DNS-enriched entries into offline parses run in this process
+        # while spawned workers start cold, breaking parity.
+        parsedmarc.IP_ADDRESS_CACHE.clear()
 
 
 class TestParallelMapParseReportFile(_ParallelTestCase):