Add a test for invalid name in default specification

test_suite/ui/invalid_name_in_default.rs

@@ -0,0 +1,11 @@
+use enumflags2::bitflags;
+
+#[bitflags(default = A | C)]
+#[repr(u8)]
+#[derive(Clone, Copy)]
+enum Test {
+    A = 1,
+    B = 2,
+}
+
+fn main() {}

test_suite/ui/invalid_name_in_default.stderr

@@ -0,0 +1,11 @@
+error[E0599]: no variant or associated item named `C` found for enum `Test` in the current scope
+ --> $DIR/invalid_name_in_default.rs:3:26
+  |
+3 | #[bitflags(default = A | C)]
+  |                          ^
+  |                          |
+  |                          variant or associated item not found in `Test`
+  |                          help: there is a variant with a similar name: `A`
+...
+6 | enum Test {
+  | --------- variant or associated item `C` not found here