Add DMTF mockup fixtures and test helpers

Unmodified files from DMTF Redfish-Publications (public-rackmount1, public-localstorage, public-telemetry), only those the tests use, each set with DMTF's BSD-3-Clause license and a COMMANDS.txt naming the source commit and request paths.

conftest.py loads them by request path and serves a mockup over mocked HTTP, with a SessionService that hands out a token, for the transport tests.

tests/fixtures/dmtf-public-telemetry/COMMANDS.txt

@@ -0,0 +1,11 @@
+Source: https://github.com/DMTF/Redfish-Publications
+Commit: 4f81814e399213c9055863ddaff42791c6b3706a
+Mockup: mockups/public-telemetry
+
+Files are unmodified copies. Each request path maps to <path below /redfish/v1>/index.json
+(the service root is ./index.json), as in the mockup and as served by Redfish-Mockup-Server.
+
+Request paths:
+GET /redfish/v1/Systems/1
+GET /redfish/v1/Systems/1/ResetActionInfo
+GET /redfish/v1/Managers/1/NetworkProtocol