[Unit tests] Use the rpaths determined for unit tests

The function that adds runtime link flags overrides RPATH settings via
`BUILD_WITH_INSTALL_RPATH`. Stop doing that for unit tests, because they
depend on host libraries built into `lib` not `lib/swift/${platformname}`.

Author: DougGregor

cmake/modules/AddSwiftUnittests.cmake

@@ -81,6 +81,7 @@ function(add_swift_unittest test_dirname)
 
   if (SWIFT_SWIFT_PARSER)
     _add_swift_runtime_link_flags(${test_dirname} "../../lib" "")
+    set_property(TARGET ${test_dirname} PROPERTY BUILD_WITH_INSTALL_RPATH OFF)
   endif()
 endfunction()