[Infra] Add `@unchecked` Sendable to `MockQuery` (#15124)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

FirebaseCombineSwift/Tests/Unit/Firestore/GetDocumentsTests.swift

@@ -20,7 +20,7 @@ import XCTest
 
 class GetDocumentsTests: XCTestCase {
   let expectationTimeout: TimeInterval = 2
-  class MockQuery: QueryFake {
+  class MockQuery: QueryFake, @unchecked Sendable {
     var mockGetDocuments: () throws -> QuerySnapshot = {
       fatalError("You need to implement \(#function) in your mock.")
     }