[llvm] [lit] Echo full RUN lines in case of external shells (PR #65267)

[via llvm-commits]

dyung wrote:

It's probably not much help, but this affected one of my build bots, and when I ran check-all manually on the machine, it died with an error "User defined signal 2":
```
PASS: libFuzzer-x86_64-default-Linux :: recommended-dictionary.test (72990 of 75935)
PASS: libFuzzer-x86_64-default-Linux :: cleanse.test (72991 of 75935)                                                                                                                                                                             
User defined signal 2
```

https://github.com/llvm/llvm-project/pull/65267