[System.Drawing.Common] In test, dispose of Metafile after Graphics

Fixes dotnet#37838

Verified using libgdiplus built with
mono/libgdiplus#671

src/libraries/System.Drawing.Common/tests/mono/System.Imaging/MetafileTest.cs

@@ -408,8 +408,8 @@ public void Measure()
                 RectangleF rect = new RectangleF(0, 0, size.Width, size.Height);
                 Region[] region = g.MeasureCharacterRanges(text, test_font, rect, sf);
                 Assert.Equal(2, region.Length);
-                mf.Dispose();
             }
+            mf.Dispose();
         }
 
         [ConditionalFact(Helpers.IsDrawingSupported)]