Add GetMempoolTx gRPC test

[ftm1000]

Motivation

In https://github.com/ZcashFoundation/zebra/pull/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.

Tasks

Add a test for:

• GetMempoolTx
• GetMempoolStream

The send transaction test populates the mempool, so we can add these extra gRPC calls to the end of that test.

Draft Implementation

PR #4537 has a draft implementation.

[ftm1000]

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[ftm1000]

Split from https://github.com/ZcashFoundation/zebra/issues/4278