Fix emacs-module-tests to work out of build tree

Fix emacs-module-tests to work out of build tree

* test/Makefile.in (test_module_dir): Build the test module library in
a subdirectory of the build directory (not the source tree).
(MODULE_CFLAGS): Fix location of emacs-module.h header file.
(test_module): Move built library out of the source tree.
* test/src/emacs-module-tests.el (mod-test-file): Locate the test module
library relative to the running Emacs executable.