Fix lightwalletd test hangs by checking process output in dedicated threads

[teor2345]

Motivation

Sometimes lightwalletd hangs, but we make the Zebra integration test pass anyway.

When this happens, we update the lightwalletd cached state with a small number of blocks, so future tests are slow, or they fail because they don't have enough blocks.

Alternative Fixes

Fix lightwalletd hangs by giving each child process a dedicated test thread to check logs.

OR

When lightwalletd hangs, restart it.

OR

When lightwalletd hangs, don't update the cached lightwalletd state

OR

In the Rust test harnesses:

• don't check that the lightwalletd cached state contains a minimum number of blocks
• increase the test timeouts to allow a full lightwalletd sync