Add fix for Xcode 12.5

[kerrmarin-lvmh]

👋

I was unable to compile this project using Xcode 12.5, and I didn't see any pull requests that fixed the problem so here's my attempt.

Steps to repro:

• Clone this repo
• Open Package.swift
• CMD+B

Compiler error on XCTest.swift:48 -

'XCTIssueReference' is not implicitly convertible to 'XCTIssue'; did you mean to use 'as' to explicitly convert?

[kylef]

Oh! They really do love to keep us on our toes.

Thanks @kerrmarin-lvmh.