internal/core/export: verify issue 349 is fixed

Export implements topological sort on fields.

Fixes #349

Change-Id: I5cae07fa99cca83d839f89a79b20fd1ac1d93303
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7043
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/testdata/eval/issue349.txtar

@@ -0,0 +1,38 @@
+-- in.cue --
+ex: {
+  "aaa": "aaa",
+  "list": "list",
+  "zzz": "zzz",
+  "AAA": "AAA",
+  "html": "html",
+  "HTML": "html",
+  "ZZZ": "ZZZ",
+  "Html": "html",
+}
+-- out/eval --
+(struct){
+  ex: (struct){
+    aaa: (string){ "aaa" }
+    list: (string){ "list" }
+    zzz: (string){ "zzz" }
+    AAA: (string){ "AAA" }
+    html: (string){ "html" }
+    HTML: (string){ "html" }
+    ZZZ: (string){ "ZZZ" }
+    Html: (string){ "html" }
+  }
+}
+-- out/compile --
+--- in.cue
+{
+  ex: {
+    aaa: "aaa"
+    list: "list"
+    zzz: "zzz"
+    AAA: "AAA"
+    html: "html"
+    HTML: "html"
+    ZZZ: "ZZZ"
+    Html: "html"
+  }
+}