* module/ice-9/session.scm (submodules): replace implementation to
use `module-submodules' instead of `module-obarray' (the latter
doesn't include submodules anymore).
* test-suite/tests/session.test: new test suite for session, checking
the exported procedures that use `submodules'.
Signed-off-by: Jose A. Ortega Ruiz <jao@gnu.org>
