Add concurrency stress tests (#21)

* Add thread safety tests

* Update ThreadSafetyTests.swift

* Update ThreadSafetyTests.swift

* Update ThreadSafetyTests.swift

Author: 0xLeif

Tests/CacheTests/ThreadSafetyTests.swift

@@ -0,0 +1,41 @@
+import XCTest
+@testable import Cache
+
+final class ThreadSafetyTests: XCTestCase {
+    func testCacheConcurrentAccess() {
+        let iterations = 1000
+        let cache = Cache<Int, Int>()
+
+        DispatchQueue.concurrentPerform(iterations: iterations) { i in
+            cache.set(value: i, forKey: i)
+            XCTAssertEqual(cache.get(i), i)
+            cache.remove(i)
+        }
+
+        XCTAssertEqual(cache.allValues.count, 0)
+    }
+
+    func testLRUCacheConcurrentAccess() {
+        let iterations = 500
+        let cache = LRUCache<Int, Int>(capacity: 500)
+
+        DispatchQueue.concurrentPerform(iterations: iterations) { i in
+            cache.set(value: i, forKey: i)
+            XCTAssertEqual(cache.get(i), i)
+        }
+
+        XCTAssertEqual(cache.allValues.count, 500)
+    }
+
+    func testExpiringCacheConcurrentAccess() {
+        let iterations = 200
+        let cache = ExpiringCache<Int, Int>(duration: .hours(1))
+
+        DispatchQueue.concurrentPerform(iterations: iterations) { i in
+            cache.set(value: i, forKey: i)
+            XCTAssertEqual(cache.get(i), i)
+        }
+
+        XCTAssertEqual(cache.allValues.count, iterations)
+    }
+}