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

[via llvm-commits]

dyung wrote:

There seems to be a race condition of sorts which has caused the test shtest-run-at-line.py to be a bit unreliable on a Windows bot:
https://lab.llvm.org/buildbot/#/builders/216

It seems like there is some extra output that is not expected that is causing the CHECK-NEXT to fail on several lines. For example:
```
             6: Command Output (stderr): 
 check:17       ^~~~~~~~~~~~~~~~~~~~~~~
             7: -- 
 next:18        ^~
             8: bash.exe: warning: could not find /tmp, please create! 
             9: RUN: at line 4: true 
 next:19        !~~~~~~~~~~~~~~~~~~~  error: match on wrong line
```
The extra line `bash.exe: warning: could not find /tmp, please create!` seems to sometimes cause the test to fail. Can the test be restructured such that it can handle this?

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