t/Makefile: check helper scripts for non-portable shell commands too

Currently only the "t[0-9][0-9][0-9][0-9]-*.sh" scripts are tested for
shell incompatibilities using the check-non-portable-shell.pl script. This
makes it easy to miss non-POSIX constructs added to one of the t/*lib*.sh
helper scripts, as they aren't automatically detected.

Fix that by adding a THELPERS variable containing all shell scripts that
aren't tests and add these to the "test-lint-shell-syntax" target too.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
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. But as this does not
include the "test-lint-shell-syntax" target added the same day in commit
c7ce70ac, it is easy to accidentally add non portable shell constructs
without noticing that when running the test suite.

Fix that by always running all lint tests unless the TEST_LINT variable is
overridden. If we add less accurate or slow tests later we could still
fall back to exclude them like 81127d74 proposed. But for now it is better
to include all lint tests until proven otherwise.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>