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, 5 insertions, 0 deletions

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..cc12c71
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,5 @@
+LISP := sbcl
+
+.PHONY: test
+test:
+	$(LISP) --load test/run.lisp