add test for suffix

Author: michiboo

Tests/SwiftAlgorithmsTests/SuffixTests.swift

@@ -0,0 +1,20 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Algorithms open source project
+//
+// Copyright (c) 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+import XCTest
+import Algorithms
+
+final class SuffixTests: XCTestCase {
+  
+  func testSuffix() {
+    let a = 0...10
+    XCTAssertEqualSequences(a.suffix(while: { $0 > 5 }), (6...10))
+}