Disable package customization of module ABI name (#1601)

This disables a portion of the changes introduced in #1471 which
specified an alternate module ABI name when building this project as a
Swift package.

Resolves rdar://170069880

### Motivation:

These changes regressed our ability to build documentation for the
package. In order to reintroduce the original fix in full, we'll need a
way to customize the ABI name of a module for the purpose of building
documentation.

### 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

Package.swift

@@ -465,7 +465,10 @@ extension Array where Element == PackageDescription.SwiftSetting {
   /// module related to Swift Testing loaded into a runner process avoids this
   /// issue.
   static func moduleABIName(_ targetName: String) -> Self {
-    [.unsafeFlags(["-module-abi-name", "\(targetName)_package"])]
+    // Workaround: Disable module ABI name customization, since it has regressed
+    // building DocC documentation (see rdar://171555540).
+//    [.unsafeFlags(["-module-abi-name", "\(targetName)_package"])]
+    []
   }
 }
 

Sources/Testing/Running/Runner.swift

@@ -81,7 +81,7 @@ 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>?
 
     /// Which iteration of the test plan is being executed.
     var iteration: Int

Sources/Testing/Support/Serializer.swift

@@ -46,8 +46,13 @@ let 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