Add test system

Write a test for the `write-xfdf` function.

Worked out how to create the test system from:

- https://lispcookbook.github.io/cl-cookbook/testing.html
- https://github.com/sjl/cl-digraph/blob/master/cl-digraph.test.asd
- https://github.com/fukamachi/dexador/blob/master/dexador-test.asd

Add the test string to the `write-xfdf` function to pass the test and
check that the test runner is set up correctly.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/package.lisp b/src/package.lisp
index d535a24..e914975 100644
--- a/src/package.lisp
+++ b/src/package.lisp
@@ -1,2 +1,4 @@
 (defpackage :xfdf
-  (:use :cl))
+  (:use :cl)
+
+  (:export #:write-xfdf))