Add Info Testable interface, implement for memory.

filter/count/gi.go

Index: filter/count/gi.go
diff --git a/filter/count/gi.go b/filter/count/gi.go
index 5524af003721ebe64f3bb40e3ce4a43bc8c67a5d..896e2862e85bb269b9a87c2a71cbe8445103e1f1 100644
--- a/filter/count/gi.go
+++ b/filter/count/gi.go
@@ -144,6 +144,10 @@ func (g *infoCounter) SignBytes(bytes []byte) (string, []byte, error) {
 	return keyName, signature, g.c.SignBytes.up(err)
 }
 
+func (g *infoCounter) Testable() info.Testable {
+	return nil

[iannucci, 2016/04/14 01:12:38]

this should call through to g.gi.Testable(), or installing this filter will
break the Testable interface.

[dnj, 2016/04/14 01:15:14]

Done.

+}
+
 // FilterGI installs a counter GlobalInfo filter in the context.
 func FilterGI(c context.Context) (context.Context, *InfoCounter) {
 	state := &InfoCounter{}