make(1): add test for the currently broken :u variable modifier

The :u modifier had been broken in var.c 1.479 from 2020.08.30.19.56.02.
The code that implements the :u modifier was well-covered in the unit
tests, except for the single line that actually deals with adjacent
duplicate words.

The "refactoring" commit that replaced brk_string with Str_Words had not
taken into account that the number of words (in ac) had to be passed to
WordList_JoinFree.  Instead, the number of words was always preserved,
and the words at the end were therefore duplicated in the result.

The fix for this bug will be in the follow-up commit.

(rillig)