Define the anonymizer test plugin at module level

Windows spawns meter processes instead of forking them, so plugin instances
must be pickled to reach them. The plugin was defined inside the test
function, which cannot be pickled, and the Windows job failed with

  AttributeError: Can't pickle local object
  'NFStreamTest.test_anonymize_absent_values.<locals>.MixedValues'

Linux and macOS fork, so the plugin was inherited rather than pickled and the
test passed there. The failure was also masked at first because the Windows
jobs were failing earlier, at the Npcap download, and never reached the tests.

Moving the class to module level makes it picklable, matching how the other
plugins used in the suite are defined.

tests.py

@@ -21,6 +21,17 @@ from nfstream import NFPlugin, NFStreamer
 from nfstream.plugins import SPLT, DHCP, FlowSlicer, MDNS
 
 
+class AnonymizerMixedValues(NFPlugin):
+    """Must remain module-level because Windows spawn pickles plugins."""
+
+    def on_init(self, packet, flow):
+        flow.udps.absent = ""
+        flow.udps.populated = "value"
+        flow.udps.zero = 0
+        flow.udps.false = False
+        flow.udps.array = np.zeros(3)
+
+
 def get_files_list(path):
     files = []
     for r, d, f in os.walk(path):
@@ -918,20 +929,10 @@ class NFStreamTest(object):
             "\n----------------------------------------------------------------------"
         )
 
-        class MixedValues(NFPlugin):
-            """Produces an absent value alongside values that only look absent."""
-
-            def on_init(self, packet, flow):
-                flow.udps.absent = ""
-                flow.udps.populated = "value"
-                flow.udps.zero = 0
-                flow.udps.false = False
-                flow.udps.array = np.zeros(3)
-
         df = NFStreamer(
             source=os.path.join("tests", "pcaps", "google_ssl.pcap"),
             n_meters=1,
-            udps=MixedValues(),
+            udps=AnonymizerMixedValues(),
         ).to_pandas(
             columns_to_anonymize=[
                 "udps.absent",