tests(commondumps): Use character counts when marking columns

[zzzyxwvut]

Continue using strwidth when calculating the position of
a column of interest, and start using setcharpos when
marking a line of interest so that paired-up marks remain
aligned columnwise across all three parts generated by
term_dumpdiff, especially when multibyte characters are
written in the line before the marked column.

[zzzyxwvut]

We can see the result of counting column positions in bytes
(rather than characters) and falling short of mark alignment
between multibyte and ASCII lines with this example routine:

cd runtime/syntax
git switch --detach v9.2.0498
git revert --no-edit -Xours f7e239bd0
make clean sh_functions_bash.sh test
../../src/vim --clean -S testdir/viewdumps.vim

# :normal `a
# :normal ``
# :normal ``

[chrisbra]

thanks