testing: improve b.Loop example
The current b.Loop example doesn't focus on the basic usage of b.Loop.
Replace this with a new example that uses (slightly) more realistic
things to demonstrate the most salient points of b.Loop.
We also move the example into an example file so that we can write a
real Benchmark function and a real function to be benchmarks, which
makes this much closer to what a user would actually write.
Updates #61515.
Change-Id: I4d830b3bfe3eb3cd8cdecef469fea0541baebb43
Reviewed-on: https://go-review.googlesource.com/c/go/+/635896
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>