Add some tests for grammars.yml

It must be sorted, because it will get re-sorted when
script/download-grammars is next run and that would clutter up diffs.
And it must not contain any duplicate scopes.

test/test_pedantic.rb

@@ -3,6 +3,7 @@ require_relative "./helper"
 class TestPedantic < Test::Unit::TestCase
   filename = File.expand_path("../../lib/linguist/languages.yml", __FILE__)
   LANGUAGES = YAML.load(File.read(filename))
+  GRAMMARS = YAML.load(File.read(File.expand_path("../../grammars.yml", __FILE__)))
 
   def test_language_names_are_sorted
     assert_sorted LANGUAGES.keys
@@ -21,6 +22,16 @@ class TestPedantic < Test::Unit::TestCase
     end
   end
 
+  def test_grammars_are_sorted
+    assert_sorted GRAMMARS.keys
+  end
+
+  def test_scopes_are_sorted
+    GRAMMARS.values.each do |scopes|
+      assert_sorted scopes
+    end
+  end
+
   def assert_sorted(list)
     list.each_cons(2) do |previous, item|
       flunk "#{previous} should come after #{item}" if previous > item