Integrate module test with normal test suite

Integrate module test with normal test suite

* test/Makefile.in (ELFILES): Exclude module test if modules aren't
configured.
(EMACS_TEST_DIRECTORY): Expand test directory so that it's set
correctly even if Emacs changes the current directory.
($(srcdir)/src/emacs-module-tests.log)
($(test_module)): Proper dependency tracking for test module.

* test/data/emacs-module/Makefile (ROOT): Adapt to new location.
Remove 'check' target and EMACS variable, which are no longer
necessary.
(SO): Change to include period.

* test/src/emacs-module-tests.el (mod-test): Use EMACS_TEST_DIRECTORY
environment variable to reliably find test data.

* configure.ac (HAVE_MODULES, MODULES_SUFFIX): Add necessary
substitutions.