runtime: add tests for addrRanges.findSucc

runtime: add tests for addrRanges.findSucc

This change adds a test suite for addrRanges.findSucc so we can change
the implementation more safely.

For #40191.

Change-Id: I14a834b6d54836cbc676eb0edb292ba6176705cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/242678
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>