[Fix Flake] Fix contextvar test issue (#38076)

`//src/python/grpcio_tests/tests/unit:_contextvars_propagation_test` is very flaky, mainly in two ways:
1. Failing with error `Error in bind for address '/tmp/grpc_fullstack_test.sock': Address already in use`.
2. Failing with timeout without any error.

#### Address already in use error
This is because we're reusing the same path for all test cases: 5011420f16/src/python/grpcio_tests/tests/unit/_contextvars_propagation_test.py (L31)

#### Timeout error
We're deleting tmp file after test is done:
5011420f16/src/python/grpcio_tests/tests/unit/_contextvars_propagation_test.py (L64-L66)

This might cause Core fail to connect to channel with error: `connect failed: addr: unix:/tmp/grpc_fullstack_test.sock error: No such file or directory`, Core will keep retrying and thus causing the test to timeout.

To make things worse, we're using multiple threads in one of the test case, leading to an even higher rate of flakiness.

This PR fix the issue by using different address for different test runs.

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Closes #38076

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/38076 from XuanWang-Amos:fix_contextvar_test 93ab2b350f
PiperOrigin-RevId: 693812629

src/python/grpcio_tests/tests/unit/_contextvars_propagation_test.py

@@ -17,7 +17,7 @@ import contextlib
 import logging
 import os
 import queue
-import sys
+import tempfile
 import threading
 import unittest
 
@@ -28,7 +28,7 @@ from tests.unit import test_common
 _SERVICE_NAME = "test"
 _UNARY_UNARY = "UnaryUnary"
 _REQUEST = b"0000"
-_UDS_PATH = "/tmp/grpc_fullstack_test.sock"
+_UDS_PATH = os.path.join(tempfile.mkdtemp(), "grpc_fullstack_test.sock")
 
 
 def _unary_unary_handler(request, context):