tests/test-check-format.t

test-copies: improve description of the C+H case

This will make its role clearer.

Differential Revision: https://phab.mercurial-scm.org/D10045

#require black test-repo

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