testing: add Elapsed method to testing.B

testing: add Elapsed method to testing.B

Elapsed returns the measured elapsed time of the benchmark,
but does not change the running state of the timer.

Fixes #43620.

Change-Id: Idd9f64c4632518eec759d2ffccbf0050d84fcc03
Reviewed-on: https://go-review.googlesource.com/c/go/+/420254
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: hopehook <hopehook@qq.com>
Reviewed-by: Bryan Mills <bcmills@google.com>