Add a test for variants with additional data

Author: meithecatte

test_suite/ui/with_fields.rs

@@ -0,0 +1,6 @@
+#[derive(Copy, Clone, enumflags2::BitFlags)]
+enum Foo {
+    Bar(u32),
+}
+
+fn main() {}

test_suite/ui/with_fields.stderr

@@ -0,0 +1,5 @@
+error: Bitflag variants cannot contain additional data
+ --> $DIR/with_fields.rs:3:8
+  |
+3 |     Bar(u32),
+  |        ^^^^^