Setup an empty unit test suite for lib.functions

I've added a dependency so if lib.macros isn't working, we can skip
all the tests in lib.functions.  Therefore we can use lib.macros in
the test code.

tests/main.lisp

@@ -21,4 +21,5 @@
 
 (define-test all
   :serial t
-  :depends-on ((cantedraw/tests/macros macro-test)))
+  :depends-on ((cantedraw/tests/macros macro-test)
+               (cantedraw/tests/functions function-test)))