Ensure that when .serialized is applied to a parameterized @test func, its test cases are serialized (#1188)

This fixes a regression introduced in the changes for #901. When
`.serialized` is applied directly to a parameterized `@Test` function,
not to a containing suite, its test cases are no longer serialized. This
PR resolves and restores the original behavior by ensuring there is a
non-`nil` scope provider returned for the test function.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Fixes rdar://154529146

Author: stmontgomery

Sources/Testing/Traits/ParallelizationTrait.swift

@@ -31,6 +31,14 @@ public struct ParallelizationTrait: TestTrait, SuiteTrait {}
 // MARK: - TestScoping
 
 extension ParallelizationTrait: TestScoping {
+  public func scopeProvider(for test: Test, testCase: Test.Case?) -> Self? {
+    // When applied to a test function, this trait should provide scope to the
+    // test function itself, not its individual test cases, since that allows
+    // Runner to correctly interpret the configuration setting to disable
+    // parallelization.
+    test.isSuite || testCase == nil ? self : nil
+  }
+
   public func provideScope(for test: Test, testCase: Test.Case?, performing function: @Sendable () async throws -> Void) async throws {
     guard var configuration = Configuration.current else {
       throw SystemError(description: "There is no current Configuration when attempting to provide scope for test '\(test.name)'. Please file a bug report at https://github.com/swiftlang/swift-testing/issues/new")

Tests/TestingTests/TestSupport/TestingAdditions.swift

@@ -103,6 +103,25 @@ extension Runner {
   ///   - fileID: The `#fileID` string whose module should be used to locate
   ///     the test function to run.
   ///   - configuration: The configuration to use for running.
+  init(
+    selecting testName: String,
+    inModuleOf fileID: String = #fileID,
+    configuration: Configuration = .init()
+  ) async {
+    let plan = await Runner.Plan(selecting: testName, inModuleOf: fileID, configuration: configuration)
+    self.init(plan: plan, configuration: configuration)
+  }
+}
+
+extension Runner.Plan {
+  /// Initialize an instance of this type that selects the free test function
+  /// named `testName` in the module specified in `fileID`.
+  ///
+  /// - Parameters:
+  ///   - testName: The name of the test function this instance should run.
+  ///   - fileID: The `#fileID` string whose module should be used to locate
+  ///     the test function to run.
+  ///   - configuration: The configuration to use for running.
   init(
     selecting testName: String,
     inModuleOf fileID: String = #fileID,
@@ -116,9 +135,7 @@ extension Runner {
 
     await self.init(configuration: configuration)
   }
-}
 
-extension Runner.Plan {
   /// Initialize an instance of this type with the specified suite type.
   ///
   /// - Parameters:

Tests/TestingTests/Traits/ParallelizationTraitTests.swift

@@ -12,8 +12,11 @@
 
 @Suite("Parallelization Trait Tests", .tags(.traitRelated))
 struct ParallelizationTraitTests {
-  @Test(".serialized trait serializes parameterized test")
-  func serializesParameterizedTestFunction() async {
+  @Test(".serialized trait serializes parameterized test", arguments: await [
+    Runner.Plan(selecting: OuterSuite.self),
+    Runner.Plan(selecting: "globalParameterized(i:)"),
+  ])
+  func serializesParameterizedTestFunction(plan: Runner.Plan) async {
     var configuration = Configuration()
     configuration.isParallelizationEnabled = true
 
@@ -33,7 +36,6 @@ struct ParallelizationTraitTests {
       }
     }
 
-    let plan = await Runner.Plan(selecting: OuterSuite.self, configuration: configuration)
     let runner = Runner(plan: plan, configuration: configuration)
     await runner.run()
 
@@ -59,3 +61,8 @@ private struct OuterSuite {
     }
   }
 }
+
+@Test(.hidden, .serialized, arguments: 0 ..< 10_000)
+private func globalParameterized(i: Int) {
+  Issue.record("PARAMETERIZED\(i)")
+}