tests: turn on test-lint by default

tests: turn on test-lint by default

The test Makefile knows about a few "lint" checks for common
errors. However, they are not enabled as part of "make test"
by default, which means that many people do not bother
running them. Since they are both quick to run and accurate
(i.e., no false positives), there should be no harm in
turning them on and helping submitters catch errors earlier.

We could just set:

  TEST_LINT = test-lint

to enable all tests. But that would be unnecessarily
annoying later on if we add slower or less accurate tests
that should not be part of the default. Instead, we name the
tests individually.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>