build: add loader path to rpath for cctest

Building on Mac OS/X as follows:

```
./configure --shared
make -j4 test
```

Results in:

```
dyld: Library not loaded: @rpath/libnode.67.dylib
  Referenced from: /Users/rubys/git/node-shared/out/Release/cctest
  Reason: image not found
make: *** [cctest] Abort trap: 6
```

This change adds the loader path to the runtime path for the `cctest` executable.

Backport-PR-URL: #25681
PR-URL: #23168
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

node.gyp

@@ -956,6 +956,11 @@
         ['OS=="solaris"', {
           'ldflags': [ '-I<(SHARED_INTERMEDIATE_DIR)' ]
         }],
+        [ 'node_shared=="true"', {
+          'xcode_settings': {
+            'OTHER_LDFLAGS': [ '-Wl,-rpath,@loader_path', ],
+          },
+        }],
       ],
     }
   ], # end targets