tests: remove make check target

tests: remove make check target

Having both --enable-tests configure option and the check make target
seems redundant and confusing. Especially since the tests won't even
run without appropriate permissions for /dev/gpiochipX anyway.

Remove the check target and the --enable-install-tests option. From now
on the tests - if enabled - are built as a regular program installed to
${prefix}/bin.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>