src/sage/parallel/map_reduce.py: fixup start_workers() test case

In commit a181883 I made this test wait a bit longer (to be more
robust under load) and added a "long time" to the corresponding
start_workers() line. But, further down, there's another test that
calls finish(), which needs the workers to be started.  Without a
"long time" on the finish(), it can fail sans --long.

To fix it, we combine the two tests that call start_workers() and
finish(), and then mark the whole thing "long time". The finish() is
supposed to delete the workers created in setup_workers() as well, so
there's not much point in trying to make the "long time" more
fine-grained than that.

Author: orlitzky

src/sage/parallel/map_reduce.py

@@ -1133,18 +1133,16 @@ def start_workers(self):
 
         TESTS::
 
+            sage: # long time
             sage: from sage.parallel.map_reduce import RESetMapReduce
             sage: def children(x):
             ....:     print(f"Starting: {x}", flush=True)
             ....:     return []
             sage: S = RESetMapReduce(roots=[1, 2], children=children)
             sage: S.setup_workers(2)
-            sage: S.start_workers(); sleep(float(5))  # long time
+            sage: S.start_workers(); sleep(float(5))
             Starting: ...
             Starting: ...
-
-        Cleanup::
-
             sage: S.finish()
         """
         if self._nprocess == 0: