tests: use progress helper
With this commit, the only in-tree caller of ui.progress() is
scmutil.progress(). That means that we could deprecate it. It also
means that we can considering inlining it in scmutil.progress.
Differential Revision: https://phab.mercurial-scm.org/D3812
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors