Defines a test function with no arguments. Test functions may call other tests, so tests may be composed. If you compose tests, you should also define a function named test-ns-hook; run-tests will call test-ns-hook instead of testing all vars. Note: Actually, the test body goes in the :test metadata on the var, and the real function (the value of the var) calls test-var on itself. When *load-tests* is false, deftest is ignored.