Add test example

.devcontainer/devcontainer.json

@@ -1,11 +1,12 @@
 {
     "name": "Swift",
-    "image": "swift:5.9",
+    "image": "swift:latest",
     "features": {
         "ghcr.io/devcontainers/features/common-utils:2": {
             "installZsh": "false",
             "username": "dev"
-        }
+        },
+        "ghcr.io/swift-server-community/swift-devcontainer-features/swift-format:0": {}
     },
     "runArgs": [
         "--cap-add=SYS_PTRACE",

Package.swift

@@ -4,11 +4,16 @@
 import PackageDescription
 
 let package = Package(
-    name: "swift-with-devcontainer",
-    targets: [
-        // Targets are the basic building blocks of a package, defining a module or a test suite.
-        // Targets can depend on other targets in this package and products from dependencies.
-        .executableTarget(
-            name: "swift-with-devcontainer"),
-    ]
+  name: "swift-with-devcontainer",
+  targets: [
+    // Targets are the basic building blocks of a package, defining a module or a test suite.
+    // Targets can depend on other targets in this package and products from dependencies.
+    .executableTarget(
+      name: "swift-with-devcontainer"
+    ),
+    .testTarget(
+      name: "swift-with-devcontainerTests",
+      dependencies: ["swift-with-devcontainer"]
+    ),
+  ]
 )

Tests/example.swift

@@ -0,0 +1,7 @@
+import XCTest
+
+class ExampleTests: XCTestCase {
+  func testAdd() {
+    XCTAssertEqual(1 + 1, 2)
+  }
+}