Merge #15990: Add tests and documentation for blocksonly

fa8ced32a6 doc: Mention blocksonly in reduce-traffic.md, unhide option (MarcoFalke)
fa320de79f test: Add test for p2p_blocksonly (MarcoFalke)
fa3872e7b4 test: Format predicate source as multiline on error (MarcoFalke)
fa1dce7329 net: Rename ::fRelayTxes to ::g_relay_txes (MarcoFalke)

Pull request description:

  This is de-facto no longer hidden

ACKs for commit fa8ced:
  jamesob:
    utACK https://github.com/bitcoin/bitcoin/commit/fa8ced32a60dea37ac169241cf9a1f708ef46c4b

Tree-SHA512: 474fbdee6cbd035ed9068a066b6056c1f909ec7520be0417820fcd1672ab3069b53f55c5147968978d9258fd3a3933fe1a9ef8e4f6e14fb6ebbd79701a0a1245

test/functional/test_runner.py

@@ -141,6 +141,7 @@ BASE_SCRIPTS = [
     'rpc_net.py',
     'wallet_keypool.py',
     'p2p_mempool.py',
+    'p2p_blocksonly.py',
     'mining_prioritisetransaction.py',
     'p2p_invalid_locator.py',
     'p2p_invalid_block.py',