Fix iterate_tests to handle non-TestSuite iterables like lists

tests/test_testsuite.py

@@ -387,6 +387,23 @@ def test_two(self):
         self.assertRaises(ValueError, suite.sort_tests)
 
 
+class TestIterateTests(TestCase):
+    def test_iterate_suite(self):
+        a = PlaceHolder("a")
+        b = PlaceHolder("b")
+        suite = unittest.TestSuite([a, b])  # type: ignore[list-item]
+        self.assertEqual([a, b], list(iterate_tests(suite)))
+
+    def test_iterate_single_test(self):
+        a = PlaceHolder("a")
+        self.assertEqual([a], list(iterate_tests(a)))
+
+    def test_iterate_list(self):
+        a = PlaceHolder("a")
+        b = PlaceHolder("b")
+        self.assertEqual([a, b], list(iterate_tests([a, b])))
+
+
 class TestSortedTests(TestCase):
     def test_sorts_custom_suites(self):
         a = PlaceHolder("a")

testtools/testsuite.py

@@ -58,13 +58,13 @@ def iterate_tests(
     test_suite_or_case: TestSuiteOrCase,
 ) -> Generator[unittest.TestCase, None, None]:
     """Iterate through all of the test cases in 'test_suite_or_case'."""
-    if isinstance(test_suite_or_case, unittest.TestSuite):
-        # It's a suite, iterate through it
+    if isinstance(test_suite_or_case, Iterable) and not isinstance(
+        test_suite_or_case, unittest.TestCase
+    ):
         for test in test_suite_or_case:
             yield from iterate_tests(test)
     else:
-        # It's a test case (could be unittest.TestCase or duck-typed)
-        yield test_suite_or_case
+        yield test_suite_or_case  # type: ignore[misc]
 
 
 class ConcurrentTestSuite(unittest.TestSuite):