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:    :argadd in empty buffer changes the buffer name. (Pavol Juhas)
Solution:   Don't re-use the current buffer when not going to edit the file.
            (closes #3397)  Do re-use the current buffer for :next.

Problem:    :argedit does not reuse an empty unnamed buffer.
Solution:   Add the BLN_CURBUF flag and fix all the side effects. (Christian
            Brabandt, closes #2713)

Problem:    Still interference between test functions.
Solution:   Clear autocommands. Wipe all buffers.  Fix tests that depend on a
            specific start context.

Problem:    Test_command_count is old style.
Solution:   Turn it into a new style test. (Naruhiko Nishino)
            Use more assert functions.