tests/test-check-rust-format.t

rust-python-testing: separated base test classes

This will allow, e.g., to change `test-rust-discovery.py` simply
by adding the appropriate base class.

#require rustfmt test-repo

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

  $ cd "$TESTDIR"/..

Warning: Keep this in sync with hghave.py
  $ RUSTFMT=$(rustup which --toolchain nightly-2021-11-02 rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done