json2code_test: choose less popular port number

The json2code test uses hardcoded port 10000, which is a more popular
port number than a randomly selected one, "because powers of 10" but
also because it's used as the default port by Azurite.

I changed this to 15789 which was selected by asking an AI assistant
to select a random port in the range 10,000-20,000 which would be a
good port number.

Closes #2391

tests/unit/json2code_test.py

@@ -32,11 +32,11 @@
 class TestJson2Code(unittest.TestCase):
     rest_api_httpd = None
     server = None
-    port = 10000
+    port = 15789
 
     @classmethod
     def setUpClass(cls):
-        args = [cls.rest_api_httpd, '--port', '10000', '--smp=2']
+        args = [cls.rest_api_httpd, '--port', f'{cls.port}', '--smp=2']
         cls.server = subprocess.Popen(args,
                                       stdin=subprocess.PIPE,
                                       stdout=subprocess.PIPE,