tests,stdlib: Add a test for JsonSerializable

Change-Id: I9e1fa6ee67f5d73d41fa9972bdb9a3da7dda8957
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58729
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed