[llvm] [lit] Echo full RUN lines in case of external shells (PR #66408)

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

I think we're past that problem now.  The remaining failure appears to be a `/` vs. `\` issue in a new test introduced by [D150987](https://reviews.llvm.org/D150987).  (It also happens to be related to `/tmp`, but I think that's a coincidence.)

https://github.com/llvm/llvm-project/pull/66408