[tests] Speed up rpc_fundrawtransaction.py

Most of the time in rpc_fundrawtransaction.py is spent waiting for
unconfirmed transactions to propagate. Net processing adds a poisson
random delay to the time it will INV transactions with a mean interval
of 5 seconds. Calls like the following:

```
self.nodes[2].sendrawtransaction(signedTx['hex'])
self.sync_all()
self.nodes[1].generate(1)
````

will therefore introduce a delay waiting for the mempools to sync.
Instead just generate the block on the node that sent the transaction:

```
self.nodes[2].sendrawtransaction(signedTx['hex'])
self.nodes[2].generate(1)
```

rpc_fundrawtransaction.py is not intended to be a test for transaction
relay, so it's ok to do this.

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