[llvm-branch-commits] [MSan] Make Test work with Internal Shell (PR #165144)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/165144

This test used a subshell which is not supported by lit's internal
shell. Rewrite it to use the readfile substitution.