tests: use $RUNTESTDIR instead of $TESTDIR in narrow-library.sh
This is done so that third party extensions can use narrow-library.sh in tests.
Differential Revision: https://phab.mercurial-scm.org/D5400
#require docutils gettext
Error: the current ro localization has some rst defects exposed by
moving pager to core. These two warnings about references are expected
until the localization is corrected.
$ $TESTDIR/check-gendoc ro
checking for parse errors
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.
gendoc.txt:58: (WARNING/2) Inline interpreted text or phrase reference start-string without end-string.