Change test_pickle_globals to not check __globals__ directly

Implementation taken from cloudpipe/cloudpickle#202 (comment)

dask/tests/test_multiprocessing.py

@@ -16,11 +16,16 @@
 def test_pickle_globals():
     """ For the function f(x) defined below, the only globals added in pickling
     should be 'np' and '__builtins__'"""
-    def f(x):
-        return np.sin(x) + np.cos(x)
+    def unrelated_function(a):
+        return np.array([a])
 
-    assert set(['np', '__builtins__']) == set(
-        _loads(_dumps(f)).__globals__.keys())
+    def my_small_function(a, b):
+        return a + b
+
+    b = _dumps(my_small_function)
+    assert b'my_small_function' in b
+    assert b'unrelated_function' not in b
+    assert b'numpy' not in b
 
 
 def bad():