fa3f9a0 test: Fix intermittent sync_blocks failures (MarcoFalke)

Pull request description:

  Fixes bitcoin#18872
  Fixes bitcoin#18737
  Fixes bitcoin#18801

  See docstring for motivation and description

ACKs for top commit:
  laanwj:
    Code review ACK fa3f9a0

Tree-SHA512: acd52d386a6849f7ff1cb1a51a439dc3c76e0e3a4dd8d22030df0ebb09b44497c61c56331ff65724b695d82d86b0ebb24608f9e637008e5dacb7676b0c448889

Summary:
Backport of core [[bitcoin/bitcoin#18873 | PR18873]].

The version check is not ported as we don't expect user to run the tests
against an older version of bitcoind.

Test Plan:
  ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8337

2488ebc test: move sync_blocks and sync_mempool functions to test_framework.py (random-zebra)
d076c81 [Test] Fix intermittent sync_blocks failures (random-zebra)
f0de789 Move g_is_mempool_loaded into CTxMemPool::m_is_loaded (Ben Woosley)
0169a75 rpc: Expose g_is_mempool_loaded via getmempoolinfo and /rest/mempool/info.json (Ben Woosley)

Pull request description:

  Backport the following PRs:
  - bitcoin#15323: rpc: Expose g_is_mempool_loaded via getmempoolinfo
  - bitcoin#18873: test: Fix intermittent sync_blocks failures
  - bitcoin#19208: test: move sync_blocks and sync_mempool functions to test_framework.py

ACKs for top commit:
  furszy:
    nice finding!, code review ACK 2488ebc.
  Fuzzbawls:
    ACK 2488ebc

Tree-SHA512: 4971a0076b4179b78c5338073a10fbf0649310c2cf3907cff1ab5c02f8dd67f5f7bb4bf524d69c554c8317bd98ce46411ff740aff676ed147525c75d83214976

Summary
---

This is a backport of bitcoin/bitcoin#18873

The changes to `feature_loadblock.py` have been dropped because we do
not have that test. The version check in `test_node.py` has also been
dropped because we do not support running tests against older versions.

Included is a test, `bchn-rpc-node-sync.py`, that just sets up two
nodes, generates 101 blocks and sends a TX. This is intended as a fast
way to test the test framework node setup and `sync_all` code.

Fixes issue Bitcoin-ABC#218

Test Plan
---

* `ninja check-functional`
* `test/functional/test_runner.py --failfast -j $(nproc) $(echo {1..256} | sed 's/[^ ]*/bchn-rpc-node-sync.py/g')`
* `test/functional/test_runner.py --failfast -j $(nproc) $(echo {1..32} | sed 's/[^ ]*/feature_reindex.py mempool_persist.py wallet_abandonconflict.py wallet_basic.py/g')`