[release-branch.go1.10] cmd/internal/test2json: support subtests containing colons

[release-branch.go1.10] cmd/internal/test2json: support subtests containing colons

The "updates" lines, such as RUN, do not contain a colon. However,
test2json looked for one anyway, meaning that it would be thrown off if
it encountered a line like:

	=== RUN   TestWithColons/[::1]

In that case, it must not use the first colon it encounters to separate
the action from the test name.

Fixes golang/go#25027

Change-Id: I82eff23e24b83dae183c0cf9f85fc5f409f51c25
Reviewed-on: https://go-review.googlesource.com/98445
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
(cherry picked from commit 0c5cfec84424bb453ccd270f4b5c439f21ccf617)
Reviewed-on: https://go-review.googlesource.com/110075
Run-TryBot: Andrew Bonventre <andybons@golang.org>