Add unit tests for base helper modules (#797)

- add unit tests for base metrics, logging, spec, parameter_spec,
  and flow modules
- add a lightweight test-only module loader so these tests can run
  without optional runtime dependencies
- fix Parameter.start/stop to accept self

trustgraph-base/trustgraph/base/parameter_spec.py

@@ -4,9 +4,9 @@ from . spec import Spec
 class Parameter:
     def __init__(self, value):
         self.value = value
-    async def start():
+    async def start(self):
         pass
-    async def stop():
+    async def stop(self):
         pass
         
 class ParameterSpec(Spec):