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.

PR-URL: #23168
Reviewed-By: Richard Lau <[email protected]>
Reviewed-By: Refael Ackermann <[email protected]>
Reviewed-By: James M Snell <[email protected]>

node.gyp

@@ -903,6 +903,11 @@
         [ 'OS=="win" and node_shared=="true"', {
           'type': 'none',
         }],
+        [ 'node_shared=="true"', {
+          'xcode_settings': {
+            'OTHER_LDFLAGS': [ '-Wl,-rpath,@loader_path', ],
+          },
+        }],
       ],
     }, # cctest
   ], # end targets