[#51] add support for expected failures in nettest

Description

Add support for expected failures to nettest and the implementation for the Pure and Client case. When no failure is expected the only accepted result is a success, when instead it is expected a success is rejected and the resulting failure needs to match the expected one.

NOTE: the possible expected failures have been left deliberately loosely defined. They should be fairly useful and still retain the ability to have more implementations in the future.

Related issue(s)

Resolves #51 (closed)

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