Add config tests

[bendtherules]

Things done :
(In tests/ base directory)

1. Add test_cases folder which contains some cases to mimic project structures ptw might face
2. Test_ini_path_collected.test_basic - Test pytest.ini location reported by plugin is correct, in the most basic situation
3. Test_ini_path_collected.test_from_inner_dir - Test pytest.ini location, when pwd is one level deeper than config file dir.
4. Test_ini_path_collected.test_first_ini_from_inner_dir - Checked from inner directory, and there are two ini files in two subsequent upper level dir. Check that ini from the just upper level dir is found.
5. Test_merge_config.test_works_alone - In merge_config func, check ini config only w/o cmdline args works.
6. Test_merge_config.test_works_with_cmdline - Check ini config with cmdline args works.
7. Test_merge_config.test_works_with_cmdline_precedence - Check ini config with cmdline args works, with cmdline precedence
8. Test_merge_config.test_works_wo_ini - Check cmdline args works, w/o config file present.
9. Test_merge_config.test_works_wo_pytest_watch_section - Check cmdline args + (pytest.ini without [pytest-watch] section) works.
10. Test_merge_config.test_works_directories_ini_option - Check ini config with directories option works, related to Allow setting <directories> argument from pytest.ini #41 . It is marked for xfail, but should pass later once it is resolved.

I have put on a experimental xfail condition for test_works_directories_ini_option test, which checks if the #41 is actually still open and if yes, then xfail. Although this probably should be removed finally, it seems useful to me as a reminder in the interim period as long as the bug is not fixed.

Helpers:

1. patched_chdir - Just like chdir, but can be used in with, sets back the old path at exit. Useful for moving around and copying files around.
2. create_temp_case - Copy a test_case folder to a tmpdir and return the location of the copied folder.
3. is_github_issue_open - Returns if a gh issue is open, useful for xfail condition.

[bendtherules]

Ready for merge.