Support tests for open bugs

[Niols]

Hello,

This PR tries to provide a way of adding test cases for open bugs. This goes by adding a .open file near an existing test case. The tests runner (tests/run) has been made aware of this change and reports failures of open tests as warnings and doesn't fail when only open tests failed.

It also contains test cases for a few open issues.

@yurug What do you think of this?