[llvm-branch-commits] [NSan] Make Tests work with Internal Shell (PR #165142)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/165142

There was one test that was using a subshell. This is not supported by
lit's internal shell. Rewrite the test to use the readfile substitution.