Add another test of js special assignment merging (#24070)

Author: sandersn

tests/cases/fourslash/jsSpecialAssignmentMerging2.ts

@@ -0,0 +1,24 @@
+/// <reference path="fourslash.ts" />
+// @noEmit: true
+// @allowJs: true
+// @checkJs: true
+
+// @Filename: b.d.ts
+//// declare namespace N {
+////   class X { }
+//// }
+// @Filename: a.js
+//// var N = {};
+//// N.X = function() { };
+// @Filename: test.js
+//// var c = N.X
+//// /*1*/
+
+// #24015
+// This failed with 13 and up on my machine, so 20 is 2**7 more than needed.
+for (let i = 0; i < 20; i++) {
+    goTo.marker('1');
+    edit.insertLine('c');
+
+    verify.getSemanticDiagnostics([])
+}