test: Add UI tests for invalid submodules

Author: Techassi

crates/stackable-versioned-macros/tests/default/fail/submodule_invalid_name.rs

@@ -0,0 +1,8 @@
+use stackable_versioned_macros::versioned;
+
+fn main() {
+    #[versioned(version(name = "v1alpha1"))]
+    mod versioned {
+        mod v1alpha2 {}
+    }
+}

crates/stackable-versioned-macros/tests/default/fail/submodule_invalid_name.stderr

@@ -0,0 +1,10 @@
+use stackable_versioned::versioned;
+
+fn main() {
+    #[versioned(version(name = "v1alpha1"))]
+    mod versioned {
+        mod v1alpha1 {
+            struct Foo;
+        }
+    }
+}