JS: disable MergeObjsMode test

This isn't implemented for JS backend objects.

testsuite/tests/driver/MergeObjsMode/A.hs

 module A where
 
+-- Don't inline otherwise A.o may not be needed by Main.o
+{-# NOINLINE a #-}
 a :: Int
 a = 42
 

testsuite/tests/driver/MergeObjsMode/B.hs

 module B where
 
+-- Don't inline otherwise B.o may not be needed by Main.o
+{-# NOINLINE b #-}
 b :: String
 b = "hello world"

testsuite/tests/driver/MergeObjsMode/all.T

 test('MergeObjsMode',
      [ extra_files(['A.hs', 'B.hs', 'Main.hs'])
+     , js_skip # Object merging isn't supported by the JS backend
      ],
      makefile_test,
      [])