test_libevreactor_shutdown: Use installed wheel in subprocess

The atexit subprocess test inserted the project root at sys.path[0], which
shadows the installed compiled wheel with the in-tree pure-Python source. Under
cibuildwheel that source lacks the libev C extension, so the import failed and
the subprocess produced no output.

Append the project path instead so the installed wheel takes precedence,
falling back to the source tree only when the driver is not installed. Also
assert the subprocess return code and include stdout/stderr in the failure
message so future subprocess import/runtime failures are easier to diagnose.

Author: sylwiaszunejko

tests/unit/io/test_libevreactor_shutdown.py

@@ -117,8 +117,11 @@ def test_shutdown_cleanup_works_with_fix(self):
 import sys
 import os
 
-# Add the driver path
-sys.path.insert(0, {driver_path!r})
+# Add the driver path as a fallback only. Append (not insert at 0) so that an
+# installed build of the driver (e.g. the compiled wheel under cibuildwheel)
+# takes precedence over the in-tree pure-Python source, which lacks the libev
+# C extension and would make the import fail.
+sys.path.append({driver_path!r})
 
 # Import and setup
 from cassandra.io import libevreactor
@@ -162,9 +165,18 @@ def test_shutdown_cleanup_works_with_fix(self):
             )
 
             output = result.stdout
+            error_output = result.stderr
             print("\n=== Subprocess Output ===")
             print(output)
             print("=== End Output ===\n")
+            print("\n=== Subprocess Error Output ===")
+            print(error_output)
+            print("=== End Error Output ===\n")
+
+            self.assertEqual(
+                result.returncode, 0,
+                "Subprocess failed\nstdout:\n{}\nstderr:\n{}".format(output, error_output)
+            )
 
             # Verify the output shows the fix is working
             self.assertIn("Global loop initialized: True", output)