Add test_expensive target to the main Makefile

Now 'make test' will run the go tests with option -test.short
and the sharness tests without any option.

And 'make test_expensive' will run the go tests without any
option and the sharness tests with option TEST_EXPENSIVE=1.

This should help fix the first part of issue #283.

License: MIT
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>