database/sql: fix flake in TestContextCancelDuringRawBytesScan

database/sql: fix flake in TestContextCancelDuringRawBytesScan

If the cancellation takes effect between Next and Scan,
then Scan returns context.Canceled, but the test wasn't
allowing this case.

The old flake reproduced easily with:

	go test -c
	stress ./sql.test -test.count=100 -test.run=TestContextCancelDuringRawBytesScan

The new test modes exercise that path directly instead of needing stress.

The new check for context.Canceled fixes the new test mode "top".

Fixes #60445.

Change-Id: I3870039a0fbe0a43c3e261b43b175ef83f818765
Reviewed-on: https://go-review.googlesource.com/c/go/+/502876
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>