Enable Library Evolution for package-based builds of the _TestDiscovery target (#1082)

This enables Library Evolution (LE) for the `_TestDiscovery` target in
Swift package-based builds. The CMake-based build is already configured
to enable LE for this target, so this is just matching behavior.

(Note that we already enable LE for several other targets which vend
public API as of #951.)

### 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

@@ -171,7 +171,7 @@ let package = Package(
       dependencies: ["_TestingInternals",],
       exclude: ["CMakeLists.txt"],
       cxxSettings: .packageSettings,
-      swiftSettings: .packageSettings
+      swiftSettings: .packageSettings + .enableLibraryEvolution()
     ),
 
     // Cross-import overlays (not supported by Swift Package Manager)