[test] Enable --wasm-tier-up when test does serialization

With a recent change, we require WebAssembly code to be tiered up to
serialize it, see https://crrev.com/c/2349290. In that CL tests were
adjusted to set the --wasm-tier-up flag when serialization was involved.
However, the test adjusted in this CL was missing, because this test
used the kExprRefNull instruction, which caused a bailout to TurboFan
anyways. With recent changes, Liftoff can compile kExprRefNull now,
and therefore causes problems.

R=thibaudm@chromium.org

Bug: v8:10852
Change-Id: I9b89f37c22f17cbf046110f3ee1c98bfea73e009
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2387574Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69648}

test/mjsunit/regress/wasm/regress-7785.js

@@ -2,7 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --allow-natives-syntax --experimental-wasm-reftypes
+// The test needs --wasm-tier-up because we can't serialize and deserialize
+// Liftoff code.
+// Flags: --allow-natives-syntax --experimental-wasm-reftypes --wasm-tier-up
 
 load("test/mjsunit/wasm/wasm-module-builder.js");