cue/testdata: add a regression test for issue 2526

This CUE caused a hang on evalv2, but correctly reports
a structural cycle on evalv3 now.

Closes #2526.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I15b0d0950bc48943cbe6c5253c82feaaa9936800
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1214546
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

cue/testdata/cycle/issue2526.txtar

@@ -0,0 +1,34 @@
+// This code caused a hang on evalv2.
+#skip-v2
+-- in.cue --
+x
+x: {
+	y
+	y: x
+}
+-- out/compile --
+--- in.cue
+{
+  〈0;x〉
+  x: {
+    〈0;y〉
+    y: 〈1;x〉
+  }
+}
+-- out/evalalpha --
+Errors:
+x.y: structural cycle
+
+Result:
+(_|_){
+  // [structural cycle]
+  x: (_|_){
+    // [structural cycle]
+    y: (_|_){
+      // [structural cycle] x.y: structural cycle
+    }
+  }
+  y: (_|_){
+    // [structural cycle]
+  }
+}