Add tests and documentation for blocksonly

Summary:

• net: Rename ::fRelayTxes to ::g_relay_txes

This helps to distinguish it from CNode::fRelayTxes and avoid bugs like
425278d17bd0edf8a3a7cc81e55016f7fd8e7726

• test: Format predicate source as multiline on error
• test: Add test for p2p_blocksonly
• doc: Mention blocksonly in reduce-traffic.md, unhide option

This is a backport of Core PR15990

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6424