Add missing wallet grpc tests

[oxarbitrage]

In #4253 we added tests for grpc wallet methods, however we left a few TODOs in the main file (zebrad/tests/common/lightwalletd/wallet_grpc_test.rs)for missing tests.

This ticket is for add them:

• Add Address stream gRPC tests #4362
• Add GetMempoolTx and GetMempoolStream gRPC tests #4350
• Add GetTreeState gRPC test #4351
  • feat(rpc): Implement z_gettreestate RPC #3990
  • ECC implementing parsing and gRPC in https://github.com/zcash/lightwalletd
  • Aditya merging those changes to https://github.com/adityapk00/lightwalletd

The stream ones should be easy, the tree state should also be easy after #3990 is in place. Mempool ones are harder as we will have to add some transaction to the mempool from the test (idea: need to use code from zebrad/tests/common/lightwalletd/send_transaction_test.rs)

[ftm1000]

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[teor2345]

There's just one sub-ticket left here, so I'm going to close this tracking issue.