Tests: Explicitly set multiprocessing start method to "fork"

By using multiprocessing's ForkProcess, the testsuite will also start
working on macOS again, even on Python 3.8+.

The background on this is that beginning with Python 3.8+, "fork" is not
the default on all platforms anymore but has been changed to "spawn" on
macOS. This method, however, apparently is not compatible with how 
multiprocessing is currently used within the test harness.

src/crate/client/test_http.py

@@ -29,10 +29,10 @@
 import random
 import traceback
 from http.server import BaseHTTPRequestHandler, HTTPServer
+from multiprocessing.context import ForkProcess
 from unittest import TestCase
 from unittest.mock import patch, MagicMock
 from threading import Thread, Event
-from multiprocessing import Process
 from decimal import Decimal
 import datetime as dt
 import urllib3.exceptions
@@ -396,7 +396,7 @@ class KeepAliveClientTest(TestCase):
 
     def __init__(self, *args, **kwargs):
         super(KeepAliveClientTest, self).__init__(*args, **kwargs)
-        self.server_process = Process(target=self._run_server)
+        self.server_process = ForkProcess(target=self._run_server)
 
     def setUp(self):
         super(KeepAliveClientTest, self).setUp()
@@ -529,8 +529,8 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.assertIsNotNone(self.request_handler)
         self.server_address = ('127.0.0.1', random.randint(65000, 65535))
-        self.server_process = Process(target=TestingHTTPServer.run_server,
-                                      args=(self.server_address, self.request_handler))
+        self.server_process = ForkProcess(target=TestingHTTPServer.run_server,
+                                          args=(self.server_address, self.request_handler))
 
     def setUp(self):
         self.server_process.start()