Fix objectives tests (#348)

Set 64 bit precision in objectives tests due to change in
#347 causing tests to
fail when run in isolation.

Author: Thomas-Christie

tests/test_objectives.py

@@ -1,4 +1,5 @@
 import jax
+from jax.config import config
 import jax.numpy as jnp
 import jax.random as jr
 import pytest
@@ -19,6 +20,9 @@
     NonConjugateMLL,
 )
 
+# Enable Float64 for more stable matrix inversions.
+config.update("jax_enable_x64", True)
+
 
 def test_abstract_objective():
     with pytest.raises(TypeError):