Enable `swift test` command

Running `swift test` at the project root was failing because there were no test targets specified in Package.swift.

Package.swift

@@ -10,5 +10,6 @@ let package = Package(
     ],
     targets: [
         .target(name: "KeychainSwift", dependencies: [], path: "Sources"),
+        .testTarget(name: "KeychainSwiftTests", dependencies: ["KeychainSwift"])
     ]
 )

Tests/KeychainSwiftTests/AccessGroupTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+@testable import KeychainSwift
 
 class AccessGroupTests: XCTestCase {
   

Tests/KeychainSwiftTests/ClearTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+@testable import KeychainSwift
 
 class ClearTests: XCTestCase {
   

Tests/KeychainSwiftTests/ConcurrencyTests.swift

@@ -6,6 +6,7 @@
 //
 
 import XCTest
+@testable import KeychainSwift
 
 class ConcurrencyTests: XCTestCase {
 

Tests/KeychainSwiftTests/KeychainSwiftPrefixedTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+@testable import KeychainSwift
 
 class KeychainWithPrefixTests: XCTestCase {
   

Tests/KeychainSwiftTests/KeychainSwiftTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+@testable import KeychainSwift
 
 class KeychainSwiftTests: XCTestCase {
   

Tests/KeychainSwiftTests/SynchronizableTests.swift

@@ -1,4 +1,5 @@
 import XCTest
+@testable import KeychainSwift
 
 class SynchronizableTests: XCTestCase {
   

Tests/KeychainSwiftTests/macOS Tests/macOS_Tests.swift

@@ -1,4 +1,5 @@
 import XCTest
+@testable import KeychainSwift
 
 class macOS_Tests: XCTestCase {