test/codegen: add initial codegen tests for integer min/max

Change-Id: I006370053748edbec930c7279ee88a805009aa0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/606976


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>