Add a new regression testing module

That module should hold all tests used when reproducing (and fixing) an
issue.

tests/test_regression.py

@@ -0,0 +1,53 @@
+# Copyright: See the LICENSE file.
+
+
+"""Regression tests related to issues found with the project"""
+
+import datetime
+import typing as T
+import unittest
+
+import factory
+
+# Example objects
+# ===============
+
+
+class Author(T.NamedTuple):
+    fullname: str
+    pseudonym: T.Optional[str] = None
+
+
+class Book(T.NamedTuple):
+    title: str
+    author: Author
+
+
+class PublishedBook(T.NamedTuple):
+    book: Book
+    published_on: datetime.date
+    countries: T.List[str]
+
+
+class FakerRegressionTests(unittest.TestCase):
+    def test_locale_issue(self):
+        """Regression test for `KeyError: 'locale'`
+
+        See #785 #786 #787 #788 #790 #796.
+        """
+        class AuthorFactory(factory.Factory):
+            class Meta:
+                model = Author
+
+            class Params:
+                unknown = factory.Trait(
+                    fullname="",
+                )
+
+            fullname = factory.Faker("name")
+
+        public_author = AuthorFactory(unknown=False)
+        self.assertIsNone(public_author.pseudonym)
+
+        unknown_author = AuthorFactory(unknown=True)
+        self.assertEqual("", unknown_author.fullname)