Add test_custom.py to CI test matrix

Author: williambdean

.github/workflows/tests.yml

@@ -142,6 +142,7 @@ jobs:
           - |
             tests/dims/distributions/test_core.py
             tests/dims/distributions/test_censored.py
+            tests/dims/distributions/test_custom.py
             tests/dims/distributions/test_scalar.py
             tests/dims/distributions/test_vector.py
             tests/dims/test_model.py