rustbuild: Add `./x.py test --no-fail-fast`

rustbuild: Add `./x.py test --no-fail-fast`

This option forwards to each `cargo test` invocation, and applies the
same logic across all test steps to keep going after failures.  At the
end, a brief summary line reports how many commands failed, if any.

Note that if a test program fails to even start at all, or if an
auxiliary build command related to testing fails, these are still left
to stop everything right away.

Fixes #40219.

Gbp-Pq: Name u-rust-1.18.0-no-fail-fast.patch