Problem:    More functions can be used as methods.
Solution:   Make various functions usable as a method.

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.

Problem:    Error for Python exception does not show useful info.
Solution:   Show the last line instead of the first one. (Ben Jackson,
            closes #4381)

Problem:    Python test doesn't wipe out hidden buffer.
Solution:   Wipe out the buffer. (Ben Jackson, closes #4189)

Problem:    E315 ml_get error when using Python and hidden buffer.
Solution:   Make sure the cursor position is valid. (Ben Jackson,
            closes #4153, closes #4154)

Problem:    Using :python sets 'pyxversion' even when not executed.
Solution:   Check the "skip" flag. (Shane Harper, closes #3995)

Problem:    Python cannot handle function name of script-local function.
Solution:   Use <SNR> instead of the special byte code. (Ozaki Kiichi, closes
            #3681)

Problem:    Preferred cursor column not set in interfaces.
Solution:   Set w_set_curswant when setting the cursor. (David Hotham,
            closes #3060)

Problem:    May get ml_get error when :pydo deletes lines or switches to
            another buffer. (Nikolai Pavlov, issue #1421)
Solution:   Check the buffer and line every time.