Extend getchaintips RPC test.

Add the capability to simulate network splits to the RPC test framework
and use it to do more extensive testing of 'getchaintips'.
Work around #5113.
Merge pull request #5132

2290ed0 Work around #5113. (Daniel Kraft)
dcb9846 Extend getchaintips RPC test. (Daniel Kraft)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>