Auto merge of #105145 - Ayush1325:sequential-remote-server, r=Mark-Simulacrum

Add batch flag to remote-test-server

When using this flag, the stdout and stderr are sent in a single batch instead of being streamed. It also used `Command::output` instead of `Command::spawn`. This is useful for targets that might support std but not threading (Eg: UEFI).

Signed-off-by: Ayush Singh <ayushsingh1325@gmail.com>

Author: bors