libstdc++: Add already-accepted <ranges> testcase [PR106320]

Although PR106320 affected only the 10 and 11 branches, and the testcase
from there is already correctly accepted on trunk and the 12 branch, we
still should add the testcase to trunk/12 too for inter-branch consistency.

	PR libstdc++/106320

libstdc++-v3/ChangeLog:

	* testsuite/std/ranges/adaptors/join.cc (test13): New test.

libstdc++-v3/testsuite/std/ranges/adaptors/join.cc

@@ -205,6 +205,18 @@ test12()
   }();
 }
 
+void
+test13()
+{
+  // PR libstdc++/106320
+  auto l = std::views::transform([](auto x) {
+    return x | std::views::transform([i=0](auto y) {
+      return y;
+    });
+  });
+  std::vector<std::vector<int>> v{{5, 6, 7}};
+  v | l | std::views::join;
+}
 int
 main()
 {
@@ -220,4 +232,5 @@ main()
   test10();
   test11();
   test12();
+  test13();
 }