Improve assert output on failure

[jku]

• Failures where the assert happened in Clientrunner.assert_metadata where not handled as pytest verbose asserts: Fix this by registering the module for assert rewriting
• assert on serialized metadata so that pytest assert gives us nice output
• Increase verbosity in Makefile so it's at same level as the action

Fixes #230, mostly

[jku]

@loosebazooka this should help a little. I can't add you as reviewer but feel free.

>       assert trusted == b"", f"Unexpected trusted role {role} content"
E       AssertionError: Unexpected trusted role timestamp content
E       assert b'{\n "signat...on": 2\n }\n}' == b''
E         
E         Full diff:
E         - b''
E         + (b'{\n "signatures": [\n  {\n   "keyid": "e04d636e13f11a220ea4142d3950ac50b8ee'
E         +  b'6af91e95effbb3bee4df114d6b4e",\n   "sig": "304402206b30d8e293d3caefb7b81a'
E         +  b'6a5c3ed5cd7ed3a1e15d8df5e0da3cef56fdd6f38f02205c5b130c1403e3fba3babdba373711'
E         +  b'648dab5154c57e8e2fe634cb04698c126e"\n  },\n  {\n   "keyid": "ec9a5050bb6c78'...
E         
E         ...Full output truncated (9 lines hidden), use '-vv' to show

tuf_conformance/client_runner.py:138: AssertionError

Note that the pytest verbosity level affects how much diff pytest spits out:

• at current verbosity level used by both Makefile and the action we get a handful of lines
• Using "pytest -vv" gets you full diff I think -- but we don't currently expose verbosity level in either makefile or the action, you can only do that by manually invoking pytest