S3: improve testing for S3Client::list_object{,_v2}
S3: improve testing for S3Client::list_object{,_v2}

• list_object: Add test
• list_object_v2: Verify the second page is a continuation

mattico/rusoto,DualSpark/rusoto,mattico/rusoto,SecurityInsanity/rusoto,rusoto/rusoto,DualSpark/rusoto,DualSpark/rust-aws,DualSpark/rust-aws,SecurityInsanity/rusoto,DualSpark/rusoto,mattico/rusoto,rusoto/rusoto,rusoto/rusoto,DualSpark/rust-aws