Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)

Problem:    New behavior of b:changedtick not tested.
Solution:   Add a few test cases. (Daniel Hahler)

Problem:    islocked('d.changedtick') does not work.
Solution:   Make it work.

Problem:    b:changedtick can be unlocked, even though it has no effect.
            (Nikolai Pavlov)
Solution:   Add a check and error E940. (closes #1496)

Problem:    Can't access b:changedtick from a dict reference.
Solution:   Make changedtick a member of the b: dict. (inspired by neovim
            #6112)