All the fuzzers have the same test, which runs the fuzzer against every
entry in the corpus. Move the test logic into a separate package.
Change-Id: I3a7e2ca75d20a5ff6d51ed9e6151629e6667684b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/212258
Reviewed-by: Joe Tsai <joetsai@google.com>
