Does Apple warn of viruses on iPhone 8 and offer a virus test?

No.

Any message that you might see about "virus detected" is a scam that is trying to trick you into buying useless software.......or worse......trying to trick you into providing your personal information.

NEVER click or tap on a link in a message like this.

No. You do not need to run an anti-virus on an iPhone. That "notice" is from an unscrupulous website trying to sell you software you don't need.

Close the window and ignore it.