Create a symbolic link in the temporary directory; fix #4642

Because the linker gives a warning on FreeBSD 12.0.

```
warning: mktemp() possibly used unsafely; consider using mkstemp()
```

1 files changed, 17 insertions, 12 deletions

diff --git a/mrbgems/mruby-io/test/file.rb b/mrbgems/mruby-io/test/file.rb
index 5f88bcee8..143096759 100644
--- a/mrbgems/mruby-io/test/file.rb
+++ b/mrbgems/mruby-io/test/file.rb
@@ -179,20 +179,25 @@ end
 
 assert('File.symlink') do
   target_name = "/usr/bin"
-  symlink_name = MRubyIOTestUtil.mktemp("test-bin-dummy-XXXXXXXX")
   if !File.exist?(target_name)
     skip("target directory of File.symlink is not found")
-  else
-    begin
-      assert_equal 0, File.symlink(target_name, symlink_name)
-      begin
-        assert_equal true, File.symlink?(symlink_name)
-      ensure
-        File.delete symlink_name
-      end
-    rescue NotImplementedError => e
-      skip e.message
-    end
+  end
+
+  begin
+    tmpdir = MRubyIOTestUtil.mkdtemp("mruby-io-test.XXXXXX")
+  rescue => e
+    skip e.message
+  end
+
+  symlink_name = "#{tmpdir}/test-bin-dummy"
+  begin
+    assert_equal 0, File.symlink(target_name, symlink_name)
+    assert_equal true, File.symlink?(symlink_name)
+  rescue NotImplementedError => e
+    skip e.message
+  ensure
+    File.delete symlink_name rescue nil
+    MRubyIOTestUtil.rmdir tmpdir rescue nil
   end
 end