Inconsistent JSON-RPC tests runs

[fedekunze]

From time to time the rpc tests CI workflow fails due to an unmarshaling issue with hex string. It'd be nice to fix it. Example failure:

=== RUN   TestEth_GetFilterChanges_Topics_XB
    rpc_test.go:693: 
        	Error Trace:	rpc_test.go:693
        	Error:      	Received unexpected error:
        	            	json: cannot unmarshal hex string of odd length into Go value of type hexutil.Bytes
        	Test:       	TestEth_GetFilterChanges_Topics_XB
``

[fedekunze]

I think we should be using string instead of hexutil.Bytes everywhere? cc: @noot

[noot]

@fedekunze yeah I think that would probably fix the issue