Add unit test for multi-node failover (cascaded) issues

Branch
------
master

Details
-------
https://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=c27d3e623f8044c456597aacabe8edb39e76e4c9

Modified Files
--------------
.../disorder/tests/MultinodeCascadeFailover.js     | 119 +++++++++++++++++++++
clustertest/disorder/tests/disorder_tests.js       |   7 +-
2 files changed, 124 insertions(+), 2 deletions(-)