[llvm] [LIT][LLVM-STRINGS]Make stdin tests shell-consistent (PR #176139)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Why exactly does swapping `echo` for `printf` fix the issue? The commit description also needs to be properly updated (not just formatting the old message with strike-through).

https://github.com/llvm/llvm-project/pull/176139