tests/test-check-pylint.t

internals: copy-edit "register" -> "registrar" in configitem docs

#require test-repo pylint hg10

Run pylint for known rules we care about.
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There should be no recorded failures; fix the codebase before introducing a
new check.

Current checks:
- W0102: no mutable default argument

  $ touch $TESTTMP/fakerc
  $ pylint --rcfile=$TESTTMP/fakerc --disable=all \
  >   --enable=W0102 --reports=no \
  >   --ignore=thirdparty \
  >   mercurial hgdemandimport hgext hgext3rd
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