[libcxx][pretty printers] Disable u16string tests

Due to reported failures in a local build.

FAIL: Something is wrong in the test framework.
Converting character sets: Invalid argument.

(was enabled in https://reviews.llvm.org/D111138)

GitOrigin-RevId: cd1bd95d8707371da0e4f75cd01669c427466931
diff --git a/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp b/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp
index 39c218e..f1cbefb 100644
--- a/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp
+++ b/test/libcxx/gdb/gdb_pretty_printer_test.sh.cpp
@@ -659,7 +659,7 @@
   string_test();
   a_namespace::string_view_test();
 
-  u16string_test();
+  //u16string_test();
   u32string_test();
   tuple_test();
   unique_ptr_test();