[tests] Add generate method to TestNode
[tests] Add generate method to TestNode

Adds a generate() method to the TestNode class in the test framework.
This method intercepts calls to generate, imports a dewterministic
private key to the node and then calls generatetoaddress to generate the
block to that address.

Note that repeated calls to importprivkey for the same private keys are
no-ops, so it's fine to call the generate() method many times.

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