Fix standalone tests failing to find angle_util.dylib on Mac

BUG=

Change-Id: I69bddf27c1b019f71382d69c03eba94a268d2025
Reviewed-on: https://chromium-review.googlesource.com/385497
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

util/util.gyp

@@ -197,6 +197,10 @@
                     [
                         '<@(util_osx_sources)',
                     ],
+                    'xcode_settings':
+                    {
+                        'DYLIB_INSTALL_NAME_BASE': '@rpath',
+                    },
                     'link_settings':
                     {
                         'libraries':