testing: remove data races so that parallel benchmarks can safely call .Fatal* and .Skip*

Protects the usages of (*common).finished with locks
to prevent data races, thus allowing benchmarks to safely invoke
.Fatal* and .Skip* concurrently.

Fixes #45526

Change-Id: I2b4846f525c426d6c7d3418f8f6c86446adbf986
Reviewed-on: https://go-review.googlesource.com/c/go/+/309572


Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>