test(helgrind): fix deferred counter suppression to match mangled name

The test-fixture-deferred-counter-callback entry from the previous commit did
not fire: valgrind matches fun: patterns against the C++-mangled symbol name
(_Z13test_callback...), so a pattern anchored at the start (fun:test_callback*)
never matches the leading mangling prefix. The other four fixture suppressions
worked precisely because they lead with * (or ...). Give this one the same
leading wildcard: fun:*test_callback*. basic and file_upload already went green
in the prior run; this closes the last deferred contexts.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tAodxYJMEY4VxCX4dk62e

Author: etr

test/valgrind-helgrind.supp

@@ -297,7 +297,7 @@
 {
    test-fixture-deferred-counter-callback
    Helgrind:Race
-   fun:test_callback*
+   fun:*test_callback*
 }
 
 # --- benign lazy cache: http_resource `Allow:` header snapshot ---------------