tests: Add regression test for recursive enum with Cow and Clone

Author: Enselic

tests/ui/traits/solver-cycles/100347-recursive-enum-cow-slice.rs

@@ -0,0 +1,11 @@
+//@ check-pass
+
+use std::borrow::Cow;
+
+#[derive(Clone)]
+enum Test<'a> {
+    Int(u8),
+    Array(Cow<'a, [Test<'a>]>),
+}
+
+fn main() {}