autotools: group test scripts in check_SCRIPTS

autotools: group test scripts in check_SCRIPTS

The check_SCRIPTS is added in Makefile-env.am to list the tests that do
not require compilation. The scripts listed in check-local and in the
TESTS variable use check_SCRIPTS instead.

The PYTHONPATH environment variable is added to Makefile-env.am and
includes the pybind directory so that python unit tests can load the
libraries from sources.

http://tracker.ceph.com/issues/6274 refs #6274

Reviewed-by: Roald J. van Loon <roaldvanloon@gmail.com>
Signed-off-by: Loic Dachary <loic@dachary.org>