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..c251306bf4d64fe5e7a067f0b78dec95f09d981c 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 g.gi.Testable()
+}
+
 // FilterGI installs a counter GlobalInfo filter in the context.
 func FilterGI(c context.Context) (context.Context, *InfoCounter) {
 	state := &InfoCounter{}