Turn off the workbench test suite's side effects after each test

that turns them on.  These have the potential to affect subsequent
tests.  The side effects include automatically setting
property values, and automatically appending additional values
to list-valued properties.

stix2/test/test_workbench.py

@@ -259,6 +259,9 @@ def test_default_creator():
     assert 'created_by_ref' not in constants.CAMPAIGN_KWARGS
     assert campaign.created_by_ref == constants.IDENTITY_ID
 
+    # turn off side-effects to avoid affecting future tests
+    set_default_creator(None)
+
 
 def test_default_created_timestamp():
     timestamp = "2018-03-19T01:02:03.000Z"
@@ -269,6 +272,9 @@ def test_default_created_timestamp():
     assert stix2.utils.format_datetime(campaign.created) == timestamp
     assert stix2.utils.format_datetime(campaign.modified) == timestamp
 
+    # turn off side-effects to avoid affecting future tests
+    set_default_created(None)
+
 
 def test_default_external_refs():
     ext_ref = ExternalReference(
@@ -281,6 +287,9 @@ def test_default_external_refs():
     assert campaign.external_references[0].source_name == "ACME Threat Intel"
     assert campaign.external_references[0].description == "Threat report"
 
+    # turn off side-effects to avoid affecting future tests
+    set_default_external_refs([])
+
 
 def test_default_object_marking_refs():
     stmt_marking = StatementMarking("Copyright 2016, Example Corp")
@@ -293,6 +302,9 @@ def test_default_object_marking_refs():
 
     assert campaign.object_marking_refs[0] == mark_def.id
 
+    # turn off side-effects to avoid affecting future tests
+    set_default_object_marking_refs([])
+
 
 def test_workbench_custom_property_object_in_observable_extension():
     ntfs = stix2.v20.NTFSExt(