tests/test-check-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 black test-repo

  $ cd $RUNTESTDIR/..
  $ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`