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 rust-lang#40219.

This was added in rust-lang#38072 but I can't recall why and AFAIK Cargo already handles
this. This was discovered through rust-lang#42146 where passing duplicate flags was
causing problems.

…umeGomez

Update various book repos for the next release.

…=alexcrichton

Reduce verbosity of build logs

This does two separate things.
 - Sets sccache logging to warn instead of info.
 - Makes tests when running for a given PR (not on auto branch) quiet. (cc @eddyb)

r? @alexcrichton

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 rust-lang#40219.

…rk-Simulacrum

rustbuild: Remove RUSTFLAGS logic in rustc shim

This was added in rust-lang#38072 but I can't recall why and AFAIK Cargo already handles
this. This was discovered through rust-lang#42146 where passing duplicate flags was
causing problems.