make spec infra more ergonomic

Signed-off-by: Onur Özkan <work@onurozkan.dev>

kaptan-spec/tests/ui/no_spec.rs

@@ -0,0 +1,5 @@
+use kaptan_spec::no_spec;
+
+no_spec!("This fixture has no governed runtime behavior.");
+
+fn main() {}