tests/test-check-rust-format.t

pull: make the new argument a keyword argument

As per feedback from Dan Villiom Podlaski Christiansen
in https://phab.mercurial-scm.org/D11674#179866

Differential Revision: https://phab.mercurial-scm.org/D11730

#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