Trac #32967: long test in tensorfield.py needs # long time

I think this probably qualifies as a "long time" test:

{{{
File "src/sage/manifolds/differentiable/tensorfield.py", line 4296, in
sage.manifolds.differentiable.tensorfield.TensorField.dalembertian
Warning, slow doctest:
    De = e.dalembertian(); De
Test ran for 19.05 s
}}}

URL: https://trac.sagemath.org/32967
Reported by: mjo
Ticket author(s): Michael Orlitzky
Reviewer(s): Dima Pasechnik

Author: Release Manager

src/sage/manifolds/differentiable/tensorfield.py

@@ -4293,20 +4293,20 @@ def dalembertian(self, metric=None):
             sage: e[1] = cos(t-z)
             sage: e.display()  # plane wave propagating in the z direction
             e = cos(t - z) ∂/∂x
-            sage: De = e.dalembertian(); De
+            sage: De = e.dalembertian(); De # long time
             Vector field Box(e) on the 4-dimensional Lorentzian manifold M
 
         The function :func:`~sage.manifolds.operators.dalembertian` from the
         :mod:`~sage.manifolds.operators` module can be used instead of the
         method :meth:`dalembertian`::
 
             sage: from sage.manifolds.operators import dalembertian
-            sage: dalembertian(e) == De
+            sage: dalembertian(e) == De # long time
             True
 
         We check that the electric field obeys the wave equation::
 
-            sage: De.display()
+            sage: De.display() # long time
             Box(e) = 0
 
         """