Increase tolerance in n_qubit_tomography_test' (#2774)

I just got CI failure with this test failing with message 
```
assert np.linalg.norm(error_rho) < 0.05
AssertionError: assert 0.055035732500366044 < 0.05
```

cirq/experiments/n_qubit_tomography_test.py

@@ -126,5 +126,5 @@ def test_agrees_with_two_qubit_state_tomography(circuit):
 
     error_rho = actual_rho - expected_rho
 
-    assert np.linalg.norm(error_rho) < 0.05
+    assert np.linalg.norm(error_rho) < 0.06
     assert np.max(np.abs(error_rho)) < 0.05