tests: Clarify dependencies between tests, 'aggregate-results' and 'clean'

tests: Clarify dependencies between tests, 'aggregate-results' and 'clean'

The Makefile targets 'aggregate-results' and 'clean' pretended to be
independent.  This is not true, of course, since aggregate-results
needs the results _before_ they are removed.

Likewise, the tests should have been run already when the results are
to be aggregated.

However, as it is legitimate to run only a few tests, and then aggregate
just those results, so another target is introduced, that depends on all
tests, then aggregates the results, and only then removes the results.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>