tests: add tests for `hg evolve --continue`
This patch adds a new test file containing tests for `hg evolve --continue`.
The test file still does not contains all the tests related to `hg evolve
--continue` as one of the case is broken which needs to be fixed first. The case
is when resolving a conflicting evolve results in empty wdir.
Next patch will fix the traceback and then a later patch will add more tests.