Improve test stability

Tests/KingfisherTests/MemoryStorageTests.swift

@@ -166,8 +166,8 @@ class MemoryStorageTests: XCTestCase {
         let expiration = StorageExpiration.seconds(0.5)
         try! storage.store(value: 1, forKey: "1", expiration: expiration)
 
-        delay(0.2) {
-            // This should extend the expiration to (0.5 + 0.2) from initially created.
+        delay(0.3) {
+            // This should extend the expiration to (0.3 + 0.5) from initially created.
             let v = try! self.storage.value(forKey: "1")
             XCTAssertEqual(v, 1)
         }
@@ -177,7 +177,7 @@ class MemoryStorageTests: XCTestCase {
             XCTAssertTrue(self.storage.isCached(forKey: "1"))
         }
         
-        delay(0.8) {
+        delay(1) {
             XCTAssertFalse(self.storage.isCached(forKey: "1"))
             exp.fulfill()
         }