testing: panic in Fuzz if the function returns a value

Otherwise, the behavior of a fuzz target that returns an error could
be confusing.

Fuzz is already documented to require a function “with no return
value”, so this fixes the implementation to match the existing
documentation.

Fixes #51222

Change-Id: I44ca7ee10960214c92f5ac066ac8484c8bb9cd6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/386175


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Nooras Saba‎ <saba@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>