Allow bootstrap to use custom test runner commands #110537

Currently it's quite involved to run x.py test for a target other than the host - often you're forced to use the remote-test-server running in the specified environment. This is sometimes required to bless platform-specific UI tests.

This workflow could be made easier for contributors with a feature similar to cargo's runner option which runs executables with the specified command. For example, in order to execute aarch64-only tests, the workflow for x86-unknown-linux-gnu developers could simply be:

• Install qemu and a cross-compilation system package
• Set this option to qemu-aarch64 -L /usr/aarch64-linux-gnu (or another libc location)
• Run x.py test --target aarch64-unknown-linux-gnu

More discussion here