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

tests/test-narrow-clone-non-narrow-server.t

@@ -31,9 +31,9 b' Verify that narrow is advertised in the '
   > print(unquote(list(sys.stdin)[1]))
   > EOF
   $ echo hello | hg -R . serve --stdio | \
-  >   "$PYTHON" unquote.py | grep narrow
+  >   "$PYTHON" unquote.py | tr ' ' '\n' | grep narrow
   narrow=v0
-  rev-branch-cache changegroupsubset exp-narrow-1 getbundle known lookup protocaps pushkey streamreqs=generaldelta,revlogv1 unbundle=HG10GZ,HG10BZ,HG10UN unbundlehash
+  exp-narrow-1
 
   $ cd ..