tests: update relnotes test for actual output
I'm unclear why I didn't see this before but I consistently see it
now. In any event, it doesn't appear to be a regression, so let's just
bake it into the test.
Differential Revision: https://phab.mercurial-scm.org/D4463
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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