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, 0 deletions

diff --git a/test/run.lisp b/test/run.lisp
new file mode 100644
index 0000000..067bcff
--- /dev/null
+++ b/test/run.lisp
@@ -0,0 +1,3 @@
+(ql:quickload "xfdf-test")
+(asdf:test-system :xfdf-test)
+(quit)