Do not use absolute tolerance of 0 in tests

This fails sometimes for values expected to be 0

brian2/tests/utils.py

@@ -5,7 +5,7 @@
 from brian2.units.fundamentalunits import have_same_dimensions
 
 
-def assert_allclose(actual, desired, rtol=4.5e8, atol=0, **kwds):
+def assert_allclose(actual, desired, rtol=4.5e8, atol=1e-9, **kwds):
     """
     Thin wrapper around numpy's `~numpy.testing.utils.assert_allclose` function. The tolerance depends on the floating
     point precision as defined by the `core.default_float_dtype` preference.