Link Swift Testing documentation in its test template

This hopefully makes it easier for new users to find relevant
documentation for our test framework!

It also matches the existing pattern where XCTest documentation is
linked in its respective test template.

Author: jerryjrchen

Sources/Workspace/InitPackage.swift

@@ -549,7 +549,7 @@ public final class InitPackage {
                     func createBuildCommand(for inputPath: URL, in outputDirectoryPath: URL, with generatorToolPath: URL) -> Command? {
                         // Skip any file that doesn't have the extension we're looking for (replace this with the actual one).
                         guard inputPath.pathExtension == "my-input-suffix" else { return .none }
-                        
+
                         // Return a command that will run during the build to generate the output file.
                         let inputName = inputPath.lastPathComponent
                         let outputName = inputPath.deletingPathExtension().lastPathComponent + ".swift"
@@ -644,7 +644,7 @@ public final class InitPackage {
             content = """
             // The Swift Programming Language
             // https://docs.swift.org/swift-book
-            // 
+            //
             // Swift Argument Parser
             // https://swiftpackageindex.com/apple/swift-argument-parser/documentation
 
@@ -731,6 +731,8 @@ public final class InitPackage {
 
                 @Test func example() async throws {
                     // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+                    // Swift Testing Documentation
+                    // https://swiftpackageindex.com/swiftlang/swift-testing/documentation
                 }
 
                 """