Fix Style/FormatString offenses

`Kernel#format` is faster and will avoid to allocate an array compared
to `String#%`.

```
IPS:
       kernel_format:  3877614.2 i/s
      string_percent:  3531475.0 i/s - 1.10x  (± 0.00) slower

Memory:
       kernel_format:        160 allocated
      string_percent:        200 allocated - 1.25x more
```

1 files changed, 6 insertions, 0 deletions

diff --git a/test/drawing/tc_vml_drawing.rb b/test/drawing/tc_vml_drawing.rb
index fa939f56..e089457e 100644
--- a/test/drawing/tc_vml_drawing.rb
+++ b/test/drawing/tc_vml_drawing.rb
@@ -16,6 +16,12 @@ class TestVmlDrawing < Test::Unit::TestCase
     assert_raise(ArgumentError) { Axlsx::VmlDrawing.new }
   end
 
+  def test_pn
+    str = @vml_drawing.pn
+
+    assert_equal("drawings/vmlDrawing1.vml", str)
+  end
+
   def test_to_xml_string
     str = @vml_drawing.to_xml_string
     doc = Nokogiri::XML(str)