context: add basic context test

context/context.go

@@ -0,0 +1,16 @@
+package context
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type Store interface {
+	Fetch() string
+}
+
+func Server(store Store) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprint(w, store.Fetch())
+	}
+}