Add swiftpm-xctest-helper rpath on macOS. (#2785)

Add an extra rpath for swiftpm-xctest-helper on macOS:
`@executable_path/../../../lib/swift/macosx`.

This fixes SR-12600: `swift test --filter`.

Co-authored-by: Anders Bertelrud <[email protected]>

Co-authored-by: Anders Bertelrud <[email protected]>

Author: dan-zheng

Package.swift

@@ -167,7 +167,10 @@ let package = Package(
         .target(
             /** Shim tool to find test names on OS X */
             name: "swiftpm-xctest-helper",
-            dependencies: []),
+            dependencies: [],
+            linkerSettings: [
+                .unsafeFlags(["-Xlinker", "-rpath", "-Xlinker", "@executable_path/../../../lib/swift/macosx"], .when(platforms: [.macOS])),
+            ]),
 
         // MARK: Additional Test Dependencies