Raise at least one error for parallel tests.

Ideally, we should grab all exceptions raised and let the user see them.  For
now, at least make sure that the primary test passes.

skimage/_shared/testing.py

@@ -215,10 +215,6 @@ def test_parallel(num_threads=2):
     num_threads : int, optional
         The number of times the function is run in parallel.
 
-    Notes
-    -----
-    This decorator does not pass the return value of the decorated function.
-
     """
 
     assert num_threads > 0
@@ -227,14 +223,21 @@ def test_parallel(num_threads=2):
         @functools.wraps(func)
         def inner(*args, **kwargs):
             threads = []
-            for i in range(num_threads):
+            for i in range(num_threads - 1):
                 thread = threading.Thread(target=func, args=args, kwargs=kwargs)
                 threads.append(thread)
             for thread in threads:
                 thread.start()
+
+            result = func(*args, **kwargs)
+
             for thread in threads:
                 thread.join()
+
+            return result
+
         return inner
+
     return wrapper