* guix/build-system/asdf.scm (asdf-build): Add a #:test-asd-file argument.
[builder]: Pass it to the build system.
(package-with-build-system)[transform]: Strip it from source systems' arguments.
* guix/build/asdf-build-system.scm (check): Pass the fully qualified path to
it on to the test-system procedure.
* guix/build/lisp-utils.scm (test-system): Load the file, or otherwise one of
the often used names for it, before running the tests. Adjust the docstring
accordingly.