[clang] [libc++][lit] Allow overriding the executor for tests (PR #66545)

[Vladimir Vereschaka via cfe-commits]

vvereschaka wrote:

@arichardson 

here is a problem with your changes when building the libraries on the windows cross toolchain builders
* https://lab.llvm.org/buildbot/#/builders/60/builds/14086
* https://lab.llvm.org/buildbot/#/builders/119/builds/15283

https://lab.llvm.org/buildbot/#/builders/60/builds/14086/steps/12/logs/stdio
```
llvm-lit.py: C:\buildbot\as-builder-1\x-armv7l\llvm-project\llvm\utils\lit\lit\TestingConfig.py:151: fatal: unable to parse config file 'C:/buildbot/as-builder-1/x-armv7l/build/runtimes/runtimes-armv7-unknown-linux-gnueabihf-bins/libunwind/test/cmake-bridge.cfg', traceback: Traceback (most recent call last):
  File "C:\buildbot\as-builder-1\x-armv7l\llvm-project\llvm\utils\lit\lit\TestingConfig.py", line 139, in load_from_path
    exec(compile(data, path, "exec"), cfg_globals, None)
  File "C:/buildbot/as-builder-1/x-armv7l/build/runtimes/runtimes-armv7-unknown-linux-gnueabihf-bins/libunwind/test/cmake-bridge.cfg", line 7
    config. 'executor="C:/Python310/python.exe" "C:/buildbot/as-builder-1/x-armv7l/llvm-project/libcxx/utils/ssh.py" --host = "ubuntu at jetson6.lab.llvm.org'"
                                                                                                                                                           ^
SyntaxError: unterminated string literal (detected at line 7)

```
would you take care of it?

https://github.com/llvm/llvm-project/pull/66545