tests/test-check-rust-format.t

requirements: introduce constants for `shared` and `relshared` requirements

We add them to `WORKING_DIR_REQUIREMENTS` too as they should be stored in
`.hg/requires` and have information about the type of working copy.

Differential Revision: https://phab.mercurial-scm.org/D8926

#require rustfmt test-repo

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

  $ cd "$TESTDIR"/..
  $ RUSTFMT=$(rustup which --toolchain nightly rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done