Make recirq/kpz/experiment_test.py deterministic (#391)

Add constant seed so that the sampler and RNG generator are deterministic.
This will fix flakiness in test_size_independence_muinf within recirq/kpz/experiment_test.py

Note that the test can still produce different results if multi-threaded pytest is used depending on the order of execution of different tests, but this will vastly reduce the number of cases tested.

Author: eliottrosenberg

recirq/kpz/experiment_test.py

@@ -20,10 +20,10 @@
 
 _CYCLES = 1
 _MU = np.inf
-_SAMPLER = cirq.Simulator()
+_SAMPLER = cirq.Simulator(seed=0)
 _TRIALS = 1000
 _REPS = 1000
-rng = np.random.default_rng()
+rng = np.random.default_rng(0)
 
 
 def test_muinf_against_analytics():