topic: add test to increase code coverage in case of multiple topics
This patch adds a test which cover the part that when we have one topic
on top of other topic, this make sure that when evolving it's boundary
are the current active topic (i.e hg stack)
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null