scripted-diff: Replace ::mempool with m_node.mempool in tests

Summary:
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1. tx pool member access (mempool followed by dot) sed --regexp-extended -i -e 's/(::)?\<g_mempool\>\.([a-zA-Z])/m_node.mempool->\2/g' $(git grep -l mempool ./src/test)
2. plain global (mempool not preceeded by dot, but followed by comma) sed --regexp-extended -i -e 's/([^\.])(::)?\<g_mempool\>,/\1*m_node.mempool,/g' $(git grep -l mempool ./src/test)

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Depends on D6436 and D6438

Partial backport of core PR17407:
https://github.com/bitcoin/bitcoin/pull/17407/commits/fa538813b1c382cf135cbf2a0cc3fa01f36964d8

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6439