testing/slogtest: check for no group with empty record
As #61067 pointed out, slog did not properly handle empty groups.
https://go.dev/cl/508436 dealt with most cases inside slog itself,
but handlers must still do a check on their own. Namely, a handler
must not output a group created by WithGroup unless the Record
has attributes.
This change adds a test to slogtest to check that case.
Fixes #61227.
Change-Id: Ibc065b6e5f6e199a41bce8332ea8c7f9d8373392
Reviewed-on: https://go-review.googlesource.com/c/go/+/508438 Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>