Fix missing trailing comma to alleviate fmt errors

Offenses:

test/test_static_file.rb:151:9: C: [Corrected] Style/TrailingCommaInLiteral: Put a comma after the last item of a multiline hash.
        "collection"    => nil
        ^^^^^^^^^^^^^^^^^^^^^^

test/test_static_file.rb

@@ -148,7 +148,7 @@ class TestStaticFile < JekyllUnitTest
         "extname"       => ".txt",
         "modified_time" => @static_file.modified_time,
         "path"          => "/static_file.txt",
-        "collection"    => nil
+        "collection"    => nil,
       }
       assert_equal expected, @static_file.to_liquid.to_h
     end