Copy more from CMake for _TestingInternals (?)

Author: grynspan

Sources/_TestDiscovery/CMakeLists.txt

@@ -16,3 +16,12 @@ target_link_libraries(_TestDiscovery PRIVATE
   _TestingInternals)
 
 _swift_testing_install_target(_TestDiscovery)
+
+if(NOT BUILD_SHARED_LIBS)
+  # When building a static library, install the internal library archive
+  # alongside the main library. In shared library builds, the internal library
+  # is linked into the main library and does not need to be installed separately.
+  get_swift_testing_install_lib_dir(STATIC_LIBRARY lib_destination_dir)
+  install(TARGETS _TestDiscovery
+    ARCHIVE DESTINATION ${lib_destination_dir})
+endif()