Add missing client tests

[matthiasgeihs]

Location

package client

Problem

We have generic tests for all the client functionality in go-perun core, but not all are used here in the Perun Ethereum Backend. In particular, the following would be important to integrate here:

• Virtual channel happy and dispute test.
• Subchannel happy and dispute test.
• Failed funding test.

Proposal

Have a look at the generic client tests in go-perun and integrate them here.