PY-23549: run files as scripts for py3 unittest (instead of "discovery" engine used for py2)

In Py3 it is possible to run script directly which is much more stable than discovery machinery
For example it supports hyphens in file names

python/testData/testRunner/env/unit/withHyphen/test-foobar.py

@@ -0,0 +1,26 @@
+#!/usr/bin/env python
+
+# from falcon import testing
+import unittest
+
+
+# class TestAPI(testing.TestCase):
+
+class TestAPI(unittest.TestCase):
+    def setUp(self):
+
+        super().setUp()
+
+
+class TestCoverage(TestAPI):
+    def test_first(self):
+
+        self.assertTrue(1 == 1)
+
+    def test_second(self):
+
+        self.assertFalse(1 == 1)
+
+
+if __name__ == '__main__':
+    unittest.main()