Revert "Add high qos for delayed blocks in tests to avoid latency on CI"

This reverts commit f7b951086adc8144d196d09d56828d47cfeb9e96.

Tests/KingfisherTests/KingfisherTestHelper.swift

@@ -100,7 +100,7 @@ func clearCaches(_ caches: [ImageCache]) {
 }
 
 func delay(_ time: Double, block: @escaping ()->()) {
-    DispatchQueue.main.asyncAfter(deadline: .now() + time, qos: .userInteractive) { block() }
+    DispatchQueue.main.asyncAfter(deadline: .now() + time) { block() }
 }
 
 extension Image {