t/Makefile: always test all lint targets when running tests

t/Makefile: always test all lint targets when running tests

Only the two targets "test-lint-duplicates" and "test-lint-executable" are
currently executed when running the test target. This was done on purpose
when the TEST_LINT variable was added in 81127d74 to avoid twisted shell
scripting by developers only to avoid false positives that might result
from the rather simple minded tests, e.g. test-lint-shell-syntax. But it
looks like it might be better to include all lint tests to help developers
to detect non portable shell constructs before the patch is sent to the
list and reviewed there.

Change the TEST_LINT variable to run all lint test unless the TEST_LINT
variable is overridden. If we hit false positives more often than helping
developers to avoid non-portable code (or add less accurate or slow tests
later) we could still fall back to exclude them like 81127d74 proposed.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>