[release-branch.go1.14] testing: capture testname on --- PASS and --- FAIL lines

This fixes an issue raised at https://github.com/golang/go/issues/38458#issuecomment-635617670
in which --- PASS and --- FAIL lines would not trigger --- CONT lines
of other tests.

Updates #38458.
For #39308.

Change-Id: I0d8cc54d682a370d0a6ea6816a11b2e462a92efe
Reviewed-on: https://go-review.googlesource.com/c/go/+/235997


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/242058


Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>