This patch extends the Python test framework with the ability to split and re-join the network "at will". It uses this to extend the test for getchaintips (but the capability may be useful for other tests as well, e. g., reorg handling).
It also fixes a problem with run_test provided by BitcoinTestFramework (which is never used anywhere, though) and adds the nodes as member to the class (which seems like a good idea to me, but I can revert that change if that's not desired).
The second commit works around #5113.
