tests: split capabilities into separate lines while searching for "narrow"

Authored by av6.

Description

tests: split capabilities into separate lines while searching for "narrow"

This test is interested only in capabilities that are related to narrow, so
let's omit everything else. Makes it easier to update other capabilities (and
"rev-branch-cache" is one of the usual patterns that are already present in
tests/common-patterns.py anyway).

Differential Revision: https://phab.mercurial-scm.org/D4678