Fix tests-clean Makefile target (bug 30545)

This patch improves tests-clean Makefile target to reliably clean
test artifacts from a build directory.  Before this patch tests-clean
missed around 3k (out of total 9k) .out and .test-result files.

Signed-off-by: Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>