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.

make(1): indent directives like .info, .if, .for consistently

Done by "pkglint -F *.mk", no manual corrections.

make(1): move tests for the :O and :Or modifiers into separate files

make(1): describe the purpose of each newly added unit test dummy

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.