print.trunc.cols shows nothing if first column is too long

[tdhock]

related to #7716

> options(datatable.print.trunc.cols=TRUE)
> options(width=10)
> data.table(x="1234567", y=1, z=2, a=3)
4 variables not shown: [x, y, z, a]
> data.table(y=1, z=2, a=3, x="1234567")
   y z a
1: 1 2 3
1 variable not shown: [x]

we see that the first table has 4 variables not shown, whereas the second has only 1.
the tables have the same contents but different column order.
we could imagine improving the print method so that it only hides x (and prints the others) in both cases.

for reference ?print.data.table says

trunc.cols: If ‘TRUE’, only the columns that can be printed in the
          console without wrapping the columns to new lines will be
          printed (similar to ‘tibbles’).

[tdhock]

I guess it would be mis-leading to print columns out of order, so I would request print out like this:

> data.table(x="1234567", y=1, z=2, a=3)
1 variable not shown: [x]
   y z a
1: 1 2 3

above is proposed new printing, 1 variable not shown above the main table print out.

below is existing functionality, not shown below the main table print out.

> data.table(y=1, z=2, a=3, x="1234567")
   y z a
1: 1 2 3
1 variable not shown: [x]

we should add a test for a table with variables not shown above and below:

> data.table(b="1234567", y=1, z=2, a=3, x="1234567")
1 variable not shown: [b]
   y z a
1: 1 2 3
1 variable not shown: [x]