Parse tests as private functions.

They actually are private; we can't export / import tests. Fixes #284.

examples/acceptance_tests/047/validators/foo.ak

@@ -0,0 +1,7 @@
+fn spend(_datum: Void, _redeemer: Void, _ctx: Void) -> Bool {
+  True
+}
+
+test foo() {
+  1 + 1 == 2
+}