add a simple tests for the do_* operator

tests/test_operators.py

@@ -0,0 +1,11 @@
+import optype
+import optype._do
+from .helpers import get_callable_members
+
+
+def test_all_public():
+    """Ensure all callables in `optype._do` are in `optype.__all__`."""
+    callables_all = get_callable_members(optype)
+    callables_do = get_callable_members(optype._do)
+
+    assert callables_all == callables_do