tests: Skip container test when lacking kernel support.

This is a followup to 96b35998e6.

* tests/containers.scm ("eval/container, non-empty load path"): Add
missing 'skip-if-unsupported' call.

tests/containers.scm

@@ -269,6 +269,7 @@
                       (lset= string=? (cons* "." ".." (map basename reqs))
                              (pk (call-with-input-file result read))))))))))
 
+(skip-if-unsupported)
 (test-assert "eval/container, non-empty load path"
   (call-with-temporary-directory
    (lambda (directory)