Only run aggregate-results over actual counts

Only run aggregate-results over actual counts

The current make target 'aggregate-results' scanned all files matching
test-results/t*-*.  Normally these are only the test counts (and the
exit values, which are ignored), but with --tee the suite also dumps
all output.  Furthermore, with --verbose t1450 contains several lines
starting with "broken link from ..." which matches the criteria used
by aggregate-results.sh.

Rename the counts output files to *.counts, and only scan those.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>