Avoid ObjC runtime warnings about duplicate class definitions for Serializer type (#1444)

This is a workaround meant to avoid Objective-C runtime warnings about
duplicate classes stemming from our internal `Serializer` type which was
added in #1390.

### Motivation:

In some usage scenarios there can be two or more copies of the testing
library loaded into a runner process. When this happens on platforms
such as Darwin which use the Objective-C runtime, this can cause
duplicate class definition warnings logged to the console because class
types and (non-generic) actor types are implemented within the
Objective-C runtime as classes. This can look something like the
following:

> objc[44611]: Class _TtC7Testing10Serializer is implemented in both
../path/to/libTesting.dylib (0x1011ac050) and
path/to/swift-testing/.build/arm64-apple-macosx/debug/swift-testingPackageTests.xctest/Contents/MacOS/swift-testingPackageTests
(0x10cc3ab88). This may cause spurious casting failures and mysterious
crashes. One of the duplicates must be removed or renamed.

### Modifications:

I considered several potential fixes, but decided the simplest and most
likely to work in all usage scenarios is to make the `Serializer` type
generic by adding an unused generic type parameter `T`.

### 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.

Author: stmontgomery

Sources/Testing/Running/Runner.swift

@@ -77,7 +77,7 @@ extension Runner {
   /// type at runtime, it may be better-suited for ``Configuration`` instead.
   private struct _Context: Sendable {
     /// A serializer used to reduce parallelism among test cases.
-    var testCaseSerializer: Serializer?
+    var testCaseSerializer: Serializer<Void>?
   }
 
   /// Apply the custom scope for any test scope providers of the traits

Sources/Testing/Support/Serializer.swift

@@ -42,8 +42,13 @@ var defaultParallelizationWidth: Int {
 /// items do not start running; they must wait until the suspended work item
 /// either returns or throws an error.
 ///
+/// The generic type parameter `T` is unused. It avoids warnings when multiple
+/// copies of the testing library are loaded into a runner process on platforms
+/// which use the Objective-C runtime, due to non-generic actor types being
+/// implemented as classes there.
+///
 /// This type is not part of the public interface of the testing library.
-final actor Serializer {
+final actor Serializer<T> {
   /// The maximum number of work items that may run concurrently.
   nonisolated let maximumWidth: Int