[bootstrap] Add rpath for swiftpm-xctest-helper on macOS.

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

Author: dan-zheng

Sources/swiftpm-xctest-helper/CMakeLists.txt

@@ -9,5 +9,10 @@
 add_executable(swiftpm-xctest-helper
   main.swift)
 
+if(CMAKE_HOST_SYSTEM_NAME STREQUAL Darwin)
+  target_link_options(swiftpm-xctest-helper PRIVATE
+    "-Xlinker -rpath -Xlinker @executable_path/../../../lib/swift/macosx")
+endif()
+
 install(TARGETS swiftpm-xctest-helper
   RUNTIME DESTINATION bin)