t/Makefile: don't define TEST_RESULTS_DIRECTORY recursively

Commit 54bb901 (t/Makefile: fix result handling with
TEST_OUTPUT_DIRECTORY - 2013-04-26) incorrectly defined
TEST_RESULTS_DIRECTORY relative to itself, when it should be relative to
TEST_OUTPUT_DIRECTORY.  Fix this.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>