[#82] Mock tests for morley-client

Description

Add simple mock tests for existing morley-client functionality (big map getter, transaction sending and contract origination)

These tests substitute real RPC calls with some simple mock responses. These responses in their turn ensure some predicates that checked in the real blockchain, e.g. chain_id, protocol name and contract counter matching. However, they don't perform any signature checks, don't actually run contracts and affect their storages.

Related issue(s)

Resolves #82 (closed)

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