tests/test-check-rust-format.t

rhg: use a release-mode executable in tests

This allows the rhg build for test-rhg.t to share compiled dependencies
such as hg-core with the hg-cpython build for other tests.

For context, my wrapper script for the typical edit-compile-test
cycle now looks like this:

    (cd rust && cargo +nightly-2020-10-04 fmt)
    (cd rust && cargo build --release -p rhg)
    make --silent local PURE=--rust
    python test/run-tests.py --local "$@"

Differential Revision: https://phab.mercurial-scm.org/D9728

#require rustfmt test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"

  $ cd "$TESTDIR"/..
  $ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done