Split source-not-error ui test for structs and enums

This eliminates potential flakiness if parallel rustc were to compile
thw two impls in different threads, affecting the order in which the two
errors are emitted.

tests/ui/source-struct-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("...")]
+pub struct ErrorStruct {
+    source: String,
+}
+
+fn main() {}