This commit fixes an RTS assertion failure when invoking
rts_clearMemory with +RTS -DS. -DS implies -DZ which asserts that free
blocks contain 0xaa as the designated garbage value. Also adds the
sanity way to rts_clearMemory test to prevent future regression.
Closes #26011.

ChatGPT Codex automatically diagnosed the issue and proposed the
initial patch in a single shot, given a GHC checkout and the following
prompt:

---

Someone is reporting the following error when attempting to use `rts_clearMemory` with the RTS option `-DS`:

```
test.wasm: internal error: ASSERTION FAILED: file rts/sm/Storage.c, line 1216

    (GHC version 9.12.2.20250327 for wasm32_unknown_wasi)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
```

What's the culprit? How do I look into this issue?

---

I manually reviewed & revised the patch, tested and submitted it.