Test: Internal exchange coverage (#1525)

* Test: Internal exchange coverage

* Tests: Rename exchange.TestInstance to Setup

This package function is either going to be imported and used as just exchange.Setup,
or more likely testexch.Setup.
This removes the Stutter of having
internal/testing/exchange.TestInstance which is implicit given the
package path

internal/testing/exchange/testdata/http.json

@@ -0,0 +1,3 @@
+{
+ "routes": null
+}