testing: implement testing.B.Loop

testing: implement testing.B.Loop

Initial implementation for testing.B.Loop,
right now the calculation of b.N are still done in the old fasion way,
as of now b.Loop is merely an alias for the old loop over b.N.

For #61515.

Change-Id: If211d0acc5f0c33df530096dceafe0b947ab0c8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/608798
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Run-TryBot: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>