Disable stdin/stdout for environment_check inferior process

To work around an address sanitizer issue on macOS where
environment_check prints a spurious stderr msg when executing,

environment_check(41292,0x113e7a600) malloc: nano zone abandoned due to inability to preallocate reserved vm space.

And TestClient::Continue() which intends to continue to exit
instead sees the stderr output streamed, and doesn't handle that
unexpected output.  Easiest to disable stdin/stdout for this
one test case to avoid this corner case issue with this TestClient.cpp
way of expecting a stop reply packet after continuing.

Differential Revision: https://reviews.llvm.org/D158237