tests: mark manifestfulltextcache as conditional on revlog store
This file is currently specific to the revlog store. Other stores
may not implement it. Mark it as such in test output.
Differential Revision: https://phab.mercurial-scm.org/D4395
manifest: use fulltextcache instead of _fulltextcache
fulltextcache is part of the documented interface and is aliased
to _fulltextcache.
Differential Revision: https://phab.mercurial-scm.org/D4394