Apparently we don't have tests for libfuzzer mode. Add some tests.
Updates #57449.
Change-Id: I813da3e71c6d6f15db31914b248db220b0b7041e
Reviewed-on: https://go-review.googlesource.com/c/go/+/459555
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
