src/sage/stats/distributions/discrete_gaussian_lattice.py: add long time

There's a loop in one of the examples in this file that samples from a
distribution until a condition is met. This already takes a long time
on my machine, but in the absence of luck it can take such a long time
that it triggers a "slow doctest" warning on the CI. Since the time
taken is random, we mark it "# long time" to be safe.

Author: orlitzky

src/sage/stats/distributions/discrete_gaussian_lattice.py

@@ -504,7 +504,7 @@ def __init__(self, B, sigma=1, c=0, r=None, precision=None, sigma_basis=False):
             ....:     add_samples(1000)
             sage: sum(counter.values())  # random
             3000
-            sage: while abs(m*f(v)*1.0/nf/counter[v] - 1.0) >= 0.1:                     # needs sage.symbolic
+            sage: while abs(m*f(v)*1.0/nf/counter[v] - 1.0) >= 0.1:  # long time, needs sage.symbolic
             ....:     add_samples(1000)
 
         If the covariance provided is not positive definite, an error is thrown::