Add missing testTarget for package init swift-testing tests (#9152)

### Motivation:
When running `swift package init --type executable
--enable-swift-testing` an example test is generated but the test target
is not created in the manifest.

### Modifications:

Add the missing test target to the manifest when these types are
initialized with swift-testing tests. This applies to the `tool` and
`executable` types.

### Result:

Packages are initialized with a test target and running `swift test`
runs the generated test successfully.

Issue: #9149

Author: plemarquand

Sources/Workspace/InitPackage.swift

@@ -300,20 +300,44 @@ public final class InitPackage {
 
                 """
                 if packageType == .executable {
+                    let testTarget: String
+                    if !options.supportedTestingLibraries.isEmpty {
+                        testTarget = """
+                                .testTarget(
+                                    name: "\(pkgname)Tests",
+                                    dependencies: ["\(pkgname)"]
+                                ),
+                        """
+                    } else {
+                        testTarget = ""
+                    }
                     param += """
                             .executableTarget(
                                 name: "\(pkgname)"
                             ),
+                    \(testTarget)
                         ]
                     """
                 } else if packageType == .tool {
+                    let testTarget: String
+                    if !options.supportedTestingLibraries.isEmpty {
+                        testTarget = """
+                                .testTarget(
+                                    name: "\(pkgname)Tests",
+                                    dependencies: ["\(pkgname)"]
+                                ),
+                        """
+                    } else {
+                        testTarget = ""
+                    }
                     param += """
                             .executableTarget(
                                 name: "\(pkgname)",
                                 dependencies: [
                                     .product(name: "ArgumentParser", package: "swift-argument-parser"),
                                 ]
                             ),
+                    \(testTarget)
                         ]
                     """
                 } else if packageType == .buildToolPlugin {

Tests/WorkspaceTests/InitTests.swift

@@ -316,6 +316,8 @@ final class InitTests: XCTestCase {
             // Verify basic file system content that we expect in the package
             let manifest = path.appending("Package.swift")
             XCTAssertFileExists(manifest)
+            let manifestContents: String = try localFileSystem.readFileContents(manifest)
+            XCTAssertMatch(manifestContents, .contains(".testTarget("))
             let testFile = path.appending("Tests").appending("FooTests").appending("FooTests.swift")
             let testFileContents: String = try localFileSystem.readFileContents(testFile)
             XCTAssertMatch(testFileContents, .contains(#"import Testing"#))
@@ -344,6 +346,8 @@ final class InitTests: XCTestCase {
             // Verify basic file system content that we expect in the package
             let manifest = path.appending("Package.swift")
             XCTAssertFileExists(manifest)
+            let manifestContents: String = try localFileSystem.readFileContents(manifest)
+            XCTAssertMatch(manifestContents, .contains(".testTarget("))
             let testFile = path.appending("Tests").appending("FooTests").appending("FooTests.swift")
             let testFileContents: String = try localFileSystem.readFileContents(testFile)
             XCTAssertMatch(testFileContents, .contains(#"import Testing"#))