encoding/json: expand and modernize TestInterfaceSet

encoding/json: expand and modernize TestInterfaceSet

Add more test cases to cover a wider range of edge cases.
Use a generic addr function to take the address of a value.
Even though redudant, explicitly include a cast to the
top-level Go type so that it is more readable what the
expected input and ouput types are.

Change-Id: I3ef68df6f1beb903ae237cd49f3dcb91e5270fe7
Reviewed-on: https://go-review.googlesource.com/c/go/+/638256
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>