Add test cases documenting CBOR's behaviour with omitted and extra fields

[jordanschalm]

This PR adds two test cases to the CBOR codec package which document the behaviour of the CBOR decoding mode used for the networking layer.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.07%. Comparing base (8c170e3) to head (f03b83e).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6880      +/-   ##
==========================================
- Coverage   41.09%   41.07%   -0.02%     
==========================================
  Files        2121     2121              
  Lines      185912   185912              
==========================================
- Hits        76395    76360      -35     
- Misses     103104   103137      +33     
- Partials     6413     6415       +2     

Flag	Coverage Δ
unittests	41.07% <ø> (-0.02%)	⬇️

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[AlexHentschel]

Very nice demonstration of behaviour

[AlexHentschel]

it would be great to use the decoding mode defined in the package

flow-go/model/encoding/cbor/codec.go

Lines 38 to 41 in e8c8347

	// NetworkDecMode is the DecMode used for decoding messages over the network.
	// It returns an error if the message contains any extra field not present in the
	// target (struct we are unmarshalling into), which prevents some classes of spamming.
	var NetworkDecMode, _ = cbor.DecOptions{ExtraReturnErrors: cbor.ExtraDecErrorUnknownField}.DecMode()

[jordanschalm]

That isn't defined on this branch, but we'll get this outcome when merging the feature branch.