Run admin/cus-tests.el tests from test suite

Run admin/cus-tests.el tests from test suite

* test/Makefile.in (SUBDIRS): Run tests in new directory "misc",
intended for tests not belonging to any one file.
* test/misc/test-custom-deps.el:
* test/misc/test-custom-libs.el:
* test/misc/test-custom-noloads.el:
* test/misc/test-custom-opts.el: New files.
* test/lisp/custom-tests.el (custom--test-local-option): Move test to
above new file test-custom-opts.el.

* admin/cus-test.el: Document running tests from regular test suite.
* test/file-organization.org (Test Files): Document new test directory
"misc" for tests not belonging to any one file.