[backport] test: Add BitcoinTestFramework::sync_* methods

Summary

This is a backport of https://github.com/bitcoin/bitcoin/pull/15773

This adds methods to the test framework that can be called by just self.sync_*().

This avoids having to import the underlying util method. Also, in the default case, where all nodes are synced this avoid having to pass self.nodes explicitly.

So the effective changes are:
from test_framework.util import sync_blocks, sync_mempools
sync_blocks(self.nodes)
self.sync_blocks()
sync_mempools(self.nodes)
self.sync_mempools()

Test plan

• ninja all check-functional-extended