[test] Make regress-12945 pass with --predictable

Waiting for a background thread to finish a task isn't going to
work when there are no background threads. Luckily, we can sidestep
the problem by compiling with Turbofan immediately, instead of
triggering dynamic tier-up through repeated execution. As a nice bonus,
this makes the test faster in non-predictable modes too.

Fixed: v8:13020
Change-Id: I2d47bc07bbde48a210c6ea59551ae16e63bdae05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3736443Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81459}

test/mjsunit/regress/wasm/regress-12945.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --experimental-wasm-gc --allow-natives-syntax
+// Flags: --no-liftoff --experimental-wasm-gc --allow-natives-syntax
 
 d8.file.execute("test/mjsunit/wasm/wasm-module-builder.js");
 
@@ -40,5 +40,4 @@ builder.addFunction("crash", kSig_i_v).addBody([
 ]).exportFunc();
 
 let instance = builder.instantiate();
-let crash = instance.exports.crash;
-while (%IsLiftoffFunction(crash)) crash();
+instance.exports.crash();