[lldb][windows] The ConPTY is not fully drained at the end of a process

`lldb\test\Shell\Register\x86-64-gp-write.test` is flaky in on [lldb-x86_64-win](https://lab.llvm.org/buildbot/#/builders/211) since https://github.com/llvm/llvm-project/pull/182302 was merged. This is likely due to a race condition where we close the pipe before we have fully drained it.

https://lab.llvm.org/buildbot/#/builders/18 also failed at the same time as https://github.com/llvm/llvm-project/pull/182302 was merged however the failure did not reproduce during the following 9 runs. I think it's safe to rule it as a fluke.