tests/test-gendoc-pt_BR.t

run-tests: outputdir also has to be changed if $TESTDIR is not $PWD

Following a18eef03d879, running run-tests.py from outside tests/ would lead to
the creation of .testtimes and test-*.t.err in $PWD instead of $TESTDIR. This
patch fixes that and updates the relevant test.

#require docutils gettext

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