Before, we reset the timer at the end of T.Parallel, which is okay
assuming that T.Parallel is the first thing in the test.

Snapshot the elapsed time at the beginning of Parallel and include it in
the total duration so that any time spent in the test before calling
Parallel is reported in the test duration as well.

Updates #12243.

Change-Id: Ieca553e1f801e16b9b6416463fa8f7fa65425185
Reviewed-on: https://go-review.googlesource.com/16989Reviewed-by: Russ Cox <rsc@golang.org>