[offthread] Enable off-thread finalization in stress variant

Increase --finalize-streaming-on-background test coverage by enabling it
on the stress_js_bg_compile_wasm_code_gc variant.

Bug: chromium:1011762
Change-Id: I0df1ae9a1e73a9ce26f84c87136da1f89e9b18b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2494929
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70733}

tools/testrunner/local/variants.py

@@ -34,6 +34,7 @@ ALL_VARIANT_FLAGS = {
   "stress": [["--stress-opt", "--no-liftoff", "--stress-lazy-source-positions"]],
   "stress_concurrent_allocation": [["--stress-concurrent-allocation"]],
   "stress_js_bg_compile_wasm_code_gc": [["--stress-background-compile",
+                                         "--finalize-streaming-on-background",
                                          "--stress-wasm-code-gc"]],
   "stress_incremental_marking": [["--stress-incremental-marking"]],
   "stress_snapshot": [["--stress-snapshot"]],