tests/test-gendoc-zh_TW.t

runtest: extract the logic that update timeout for slow tests

We want to use the "slow" test logic not only for "#require" clauses but also in
"#if" clauses. For this to be useful we need the logic to bump the timeout in at
least two spots. The first step it to factor it out.

#require docutils gettext

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