[libcxx] [test] Create symlink_to_dir as the right kind, as a directory symlink

[mstorsjo]

Summary

This was missed in rG5c39eebc126dca73c086562bbd9598d4d4c7c197.

libcxx/test/support/filesystem_test_helper.h

static_test_env() {
         env_.create_dir("dir1/dir2/dir3");
         env_.create_file("dir1/dir2/dir3/file5");
         env_.create_file("dir1/dir2/file4");
         env_.create_symlink("dir3", "dir1/dir2/symlink_to_dir3", false, true);
         env_.create_file("dir1/file1");
         env_.create_file("dir1/file2", 42);
         env_.create_file("empty_file");
         env_.create_file("non_empty_file", 42);
-        env_.create_symlink("dir1", "symlink_to_dir", false);
+        env_.create_symlink("dir1", "symlink_to_dir", false, true);
         env_.create_symlink("empty_file", "symlink_to_empty_file", false);
     }
 
     const fs::path Root = env_.test_root;
 
     fs::path makePath(fs::path const& p) const {
         // env_path is expected not to contain symlinks.
         fs::path const& env_path = Root;