tests/test-gendoc-sv.t

tests: fix deprecation warning about regex flags not at beginning of expr

This may only show up when running the tests under python3.6+. Currently the
only test that does this is test-patchbomb-tls.t, and it only uses (?i), so
that's all that's handled at the moment.

Differential Revision: https://phab.mercurial-scm.org/D7552

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors