If test case framing appears in ordinary test output,
then test2json can get confused. If the fake framing is being
saved with t.Logf/t.Errorf/etc then we can already
distinguish it from real framing, and the code did.
It just forgot to write that framing as output (1-line fix).

If the fake framing is being generated by printing directly
to stdout/stderr, then test2json will simply get confused.
There's not a lot to do at that point (maybe it's even a feature).

Fixes #23036.

Change-Id: I29449c7ace304172b89d8babe23de507c0500455
Reviewed-on: https://go-review.googlesource.com/86238
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>