Fix nested multiprocessing in test

This commit fixes the error:

```
[2020-08-20 06:00:35,606]  INFO - FAIL: test_visible_devices_set_after_import (numba.cuda.tests.cudadrv.test_runtime.TestRuntime)
[2020-08-20 06:00:35,606]  INFO - ----------------------------------------------------------------------
[2020-08-20 06:00:35,606]  INFO - Traceback (most recent call last):
[2020-08-20 06:00:35,606]  INFO -   File "<path>\testenv_b522e3de-5976-43cf-ae68-63c7d7c1f2a3\lib\site-packages\numba\cuda\tests\cudadrv\test_runtime.py", line 51, in test_visible_devices_set_after_import
[2020-08-20 06:00:35,606]  INFO -     p.start()
[2020-08-20 06:00:35,606]  INFO -   File "<path>\envs\testenv_b522e3de-5976-43cf-ae68-63c7d7c1f2a3\lib\multiprocessing\process.py", line 118, in start
[2020-08-20 06:00:35,606]  INFO -     assert not _current_process._config.get('daemon'), \
[2020-08-20 06:00:35,606]  INFO - AssertionError: daemonic processes are not allowed to have children
```

by putting `test_visible_devices_set_after_import` in its own test class
with a `SerialMixin`.

numba/cuda/tests/cudadrv/test_runtime.py

@@ -2,7 +2,7 @@
 import os
 from numba.core import config
 from numba.cuda.cudadrv.runtime import runtime
-from numba.cuda.testing import unittest
+from numba.cuda.testing import unittest, SerialMixin
 
 
 def set_visible_devices_and_check(q):
@@ -26,6 +26,8 @@ def test_get_version(self):
                                   (10, 1), (10, 2), (11, 0))
         self.assertIn(runtime.get_version(), supported_versions)
 
+
+class TestVisibleDevices(unittest.TestCase, SerialMixin):
     def test_visible_devices_set_after_import(self):
         # See Issue #6149. This test checks that we can set
         # CUDA_VISIBLE_DEVICES after importing Numba and have the value