t/Makefile: Use $(sort ...) explicitly where needed

t/Makefile: Use $(sort ...) explicitly where needed

Starting from GNU Make 3.82 $(wildcard ...) no longer sorts the result
(from NEWS):

    * WARNING: Backward-incompatibility!
      Wildcards were not documented as returning sorted values, but the results
      have been sorted up until this release..  If your makefiles require sorted
      results from wildcard expansions, use the $(sort ...)  function to request
      it explicitly.

    http://repo.or.cz/w/make.git/commitdiff/2a59dc32aaf0681dec569f32a9d7ab88a379d34f

I usually watch test progress visually, and if tests are sorted, even
with make -j4 they go more or less incrementally by their t number. On
the other side, without sorting, tests are executed in seemingly random
order even for -j1. Let's please maintain sane tests order for perceived
prettyness.

Another note is that in GNU Make sort also works as uniq, so after sort
being removed, we might expect e.g. $(wildcard *.sh a.*) to produce
duplicates for e.g. "a.sh". From this point of view, adding sort could
be seen as hardening t/Makefile from accidentally introduced dups.

It turned out that prevous releases of GNU Make did not perform full
sort in $(wildcard), only sorting results for each pattern, that's why
explicit sort-as-uniq is relevant even for older makes.

Signed-off-by: Kirill Smelkov <kirr@navytux.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>