kbfs_ops_test: wait on prefetching for more specific nodes
kbfs_ops_test: wait on prefetching for more specific nodes

Always waiting for rootNode and aNode to finish doesn't make sense,
since there exists a path in the test "a/b2" that is never prefetched
by design.

Issue: KBFS-3840

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client