patch 8.0.0250: virtcol() does not work well for multi-byte characters

patch 8.0.0250: virtcol() does not work well for multi-byte characters

Problem:    When virtcol() gets a column that is not the first byte of a
            multi-byte character the result is unpredictable. (Christian
            Ludwig)
Solution:   Correct the column to the first byte of a multi-byte character.
            Change the utf-8 test to new style.