testing: increment tempDirSeq non-atomically

testing: increment tempDirSeq non-atomically

It's unnecessary to to call atomic.AddInt32 since there is a mutex lock.

Change-Id: I31fcece17c34f99a95772d744aebd3f6a8cf1d23
Reviewed-on: https://go-review.googlesource.com/c/go/+/426081
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>