[compiler-rt] [scudo] Add -Wconversion for tests and clean-up warnings. (PR #66147)

[Mitch Phillips via llvm-commits]

https://github.com/hctim commented:

Same comment in a few other places, would you be able to change the char-based magic values to something without the hex prefix (like `'A'`)?

https://github.com/llvm/llvm-project/pull/66147