Add an initial test

Along with the buildsystem changes to make `make check` work.

tests/model-derivation.scm

@@ -0,0 +1,18 @@
+(define-module (test-model-derivation)
+  #:use-module (srfi srfi-64)
+  #:use-module (guix-data-service database)
+  #:use-module (guix-data-service model derivation))
+
+(test-begin "test-model-derivation")
+
+(with-postgresql-connection
+ (lambda (conn)
+   (test-equal "valid-systems"
+     '()
+     (valid-systems conn))
+
+   (test-equal "count-derivations"
+     '("0")
+     (count-derivations conn))))
+
+(test-end)