Cherry pick test for API client context manager

The implementation and tests were already picked up by the upstream OpenAPI
generator [here](OpenAPITools/openapi-generator#5094).
Patching in the tests here for correctness and clarity.

Reference: kubernetes-client#1073

Signed-off-by: Nabarun Pal <pal.nabarun95@gmail.com>

kubernetes/test/test_api_client.py

@@ -0,0 +1,25 @@
+# coding: utf-8
+
+
+import atexit
+import weakref
+import unittest
+
+import kubernetes
+
+
+class TestApiClient(unittest.TestCase):
+
+    def test_context_manager_closes_threadpool(self):
+        with kubernetes.client.ApiClient() as client:
+            self.assertIsNotNone(client.pool)
+            pool_ref = weakref.ref(client._pool)
+            self.assertIsNotNone(pool_ref())
+        self.assertIsNone(pool_ref())
+
+    def test_atexit_closes_threadpool(self):
+        client = kubernetes.client.ApiClient()
+        self.assertIsNotNone(client.pool)
+        self.assertIsNotNone(client._pool)
+        atexit._run_exitfuncs()
+        self.assertIsNone(client._pool)