Add test to check function signature matches init return type

firmware/defmt-test/macros/tests/ui/test-has-non-empty-signature.rs

@@ -0,0 +1,14 @@
+fn main() {}
+
+#[defmt_test_macros::tests]
+mod tests {
+    #[init]
+    fn init() {
+        // empty
+    }
+
+    #[test]
+    fn test(arg: &mut u8) {
+        assert!(true);
+    }
+}

firmware/defmt-test/macros/tests/ui/test-has-non-empty-signature.stderr

@@ -0,0 +1,5 @@
+error: no state was initialized by `#[init]`; signature must be `fn()`
+  --> tests/ui/test-has-non-empty-signature.rs:11:8
+   |
+11 |     fn test(arg: &mut u8) {
+   |        ^^^^