Add test for packaging.util.set_platform (#13974).

Patch by Tshepang Lekhonkhobe.
diff --git a/Lib/packaging/tests/test_util.py b/Lib/packaging/tests/test_util.py
index 737503e..7f7ed18 100644
--- a/Lib/packaging/tests/test_util.py
+++ b/Lib/packaging/tests/test_util.py
@@ -173,6 +173,11 @@
         sys.stdout = self.old_stdout
         sys.stderr = self.old_stderr
 
+    def test_set_platform(self):
+        self.addCleanup(util.set_platform, util.get_platform())
+        util.set_platform("fake")
+        self.assertEqual("fake", util.get_platform())
+
     def test_convert_path(self):
         # linux/mac
         os.sep = '/'