tests/test-check-format.t

nodemap: check that a simple lookup works fine

I realised the code was not exercised much.

Differential Revision: https://phab.mercurial-scm.org/D8180

#require black

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