[mjsunit/wasm] Reuse WebAssembly.Memory objects in stress test

In the atomics stress, the search for sequential sequences creates
lots of new WebAssembly.Memory objects. This memory pressure is not
central to this test, so reuse the same memory to make them less
flaky.

R=mstarzinger@chromium.org

Change-Id: I8d135e7b82d572cb1df38f37a4e2f6393f6b2e05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1697247Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62644}