[release-branch.go1.14] testing: reformat test chatty output

In #24929, we decided to stream chatty test output. It looks like,

foo_test.go:138: TestFoo/sub-1: hello from subtest 1
foo_test.go:138: TestFoo/sub-2: hello from subtest 2

In this CL, we refactor the output to be grouped by === CONT lines, preserving
the old test-file-before-log-line behavior:

=== CONT TestFoo/sub-1
    foo_test.go:138 hello from subtest 1
=== CONT TestFoo/sub-2
    foo_test.go:138 hello from subtest 2

This should remove a layer of verbosity from tests, and make it easier to group
together related lines. It also returns to a more familiar format (the
pre-streaming format), whilst still preserving the streaming feature.

Updates #38458.
Fixes #39308.

Change-Id: Iaef94c580d69cdd541b2ef055aa004f50d72d078
Reviewed-on: https://go-review.googlesource.com/c/go/+/229085


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/242057


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