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 test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint