[CI] Fail subprocess tests with root-cause error (#23795)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/conftest.py

@@ -1,5 +1,15 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+# ruff: noqa
+
+from tblib import pickling_support
+
+# Install support for pickling exceptions so that we can nicely propagate
+# failures from tests running in a subprocess.
+# This should be run before any custom exception subclasses are defined.
+pickling_support.install()
+
 import http.server
 import json
 import math