[release-branch.go1.15] testing: treat PAUSE lines as changing the active test name

We could instead fix cmd/test2json to treat PAUSE lines as *not*
changing the active test name, but that seems like it would be more
confusing to humans, and also wouldn't fix tools that parse output
using existing builds of cmd/test2json.

Fixes #40849
Updates #40657

Change-Id: I937611778f5b1e7dd1d6e9f44424d7e725a589ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/248727


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
(cherry picked from commit cdc77d34)
Reviewed-on: https://go-review.googlesource.com/c/go/+/249097
TryBot-Result: Gobot Gobot <gobot@golang.org>