use nested module.mk include files for test subdirectories * test/bug-35448/module.mk, test/bug-36025/module.mk, test/bug-38236/module.mk, test/bug-38691/module.mk, test/bug-44940/module.mk, test/class-concat/module.mk, test/classdef/module.mk, test/classes/module.mk, test/ctor-vs-method/module.mk, test/fcn-handle-derived-resolution/module.mk, test/nest/module.mk: New files. * test/module.mk: Include them.