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_grammars.rb

@@ -0,0 +1,13 @@
+require_relative "./helper"
+
+class TestGrammars < Test::Unit::TestCase
+  def setup
+    @grammars = YAML.load(File.read(File.expand_path("../../grammars.yml", __FILE__)))
+  end
+
+  def test_no_duplicate_scopes
+    scopes = @grammars.values.flatten
+    duplicates = scopes.group_by { |s| s }.select { |k, v| v.length > 1 }.map(&:first)
+    assert duplicates.empty?, "The following scopes appear in grammars.yml more than once:\n#{duplicates.sort.join("\n")}"
+  end
+end