tests/make: fix test for the variable modifier ':On'

The variable modifier ':On' sorts words numerically. If these words are
not numeric at all, they get assigned the numeric value 0. Internally,
':On' uses qsort for sorting the words. Since qsort is not specified to
use a stable sorting algorithm, the test data must only use words that
either are written in the same way or that are numerically different.

The test varmod-order failed this requirement by trying to numerically
sort a list of non-numeric words. This led to different results on BSD
and Ubuntu, as could be expected.

To fix the tests, distinguish between words and numbers in the tests.
While here, clean up the tests for all variants of the variable modifier
':O'.

Found by sjg on Ubuntu.

tests/make: split tests for the variable modifier ':O'

The tests for parse errors are now in varmod-order, which lets the other
tests focus on the desired behavior of the modifiers.