Enable Library Evolution in package builds for public library targets (#951)

This modifies `Package.swift` to enable Library Evolution for builds of
the package.

### Motivation:

I recently landed a change (#931) which passed our project-level CI but
later failed in Swift CI. The difference ended up being due to the
latter building with Library Evolution (LE) enabled, whereas our
project-level CI builds via SwiftPM and does not enable LE. The change
was reverted (#950) but this revealed a gap in our testing strategy. We
should always build these targets with LE enabled.

### 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://144655439

Author: stmontgomery

Package.swift

@@ -55,7 +55,9 @@ let package = Package(
       ],
       exclude: ["CMakeLists.txt", "Testing.swiftcrossimport"],
       cxxSettings: .packageSettings,
-      swiftSettings: .packageSettings,
+      swiftSettings: .packageSettings + [
+        .enableLibraryEvolution(),
+      ],
       linkerSettings: [
         .linkedLibrary("execinfo", .when(platforms: [.custom("freebsd"), .openbsd]))
       ]
@@ -114,7 +116,9 @@ let package = Package(
         "Testing",
       ],
       path: "Sources/Overlays/_Testing_CoreGraphics",
-      swiftSettings: .packageSettings
+      swiftSettings: .packageSettings + [
+        .enableLibraryEvolution(),
+      ]
     ),
     .target(
       name: "_Testing_Foundation",
@@ -123,7 +127,12 @@ let package = Package(
       ],
       path: "Sources/Overlays/_Testing_Foundation",
       exclude: ["CMakeLists.txt"],
-      swiftSettings: .packageSettings
+      swiftSettings: .packageSettings + [
+        // The Foundation module only has Library Evolution enabled on Apple
+        // platforms, and since this target's module publicly imports Foundation,
+        // it can only enable Library Evolution itself on those platforms.
+        .enableLibraryEvolution(applePlatformsOnly: true),
+      ]
     ),
   ],
 
@@ -186,6 +195,18 @@ extension Array where Element == PackageDescription.SwiftSetting {
   }
 }
 
+extension PackageDescription.SwiftSetting {
+  /// Create a Swift setting which enables Library Evolution, optionally
+  /// constraining it to only Apple platforms.
+  ///
+  /// - Parameters:
+  ///   - applePlatformsOnly: Whether to constrain this setting to only Apple
+  ///     platforms.
+  static func enableLibraryEvolution(applePlatformsOnly: Bool = false) -> Self {
+    unsafeFlags(["-enable-library-evolution"], .when(platforms: applePlatformsOnly ? [.macOS, .iOS, .macCatalyst, .watchOS, .tvOS, .visionOS] : []))
+  }
+}
+
 extension Array where Element == PackageDescription.CXXSetting {
   /// Settings intended to be applied to every C++ target in this package.
   /// Analogous to project-level build settings in an Xcode project.

Sources/Testing/Running/Configuration.swift

@@ -218,7 +218,7 @@ public struct Configuration: Sendable {
   /// property is pre-configured. Otherwise, the default value of this property
   /// records an issue indicating that it has not been configured.
   @_spi(Experimental)
-  public var exitTestHandler: ExitTest.Handler = { _ in
+  public var exitTestHandler: ExitTest.Handler = { exitTest in
     throw SystemError(description: "Exit test support has not been implemented by the current testing infrastructure.")
   }
 #endif