Use @main in Mockolo target

[yutailang0119]

Mockolo support Swift 5.4+, so @main can be used

• https://github.com/uber/mockolo/commit/092191872fa3dbdcd0b126dde49fc9eff6c19f35

Related

• https://github.com/uber/mockolo/pull/143
• https://github.com/apple/swift-evolution/blob/main/proposals/0294-package-executable-targets.md

[uhooi]

LGTM :)