Test comment highlighting in Scala

Turns out that we don't need queries for comments, as we're already
using the is_extra() method for both parsing and highlighting.

Closes #120

sample_files/scala_after.scala

@@ -1,10 +1,12 @@
 package foo
 
+// A Scala program.
 import foo.Baz
 
 class Foo {
 
   private def blah(): Other {
+    /* foo */
     throw new Exception("after");
   }
 

sample_files/scala_before.scala

@@ -1,10 +1,12 @@
 package foo
 
+// A Scala program.
 import foo.Bar
 
 class Foo {
 
   def blah(): Int {
+    /* foo */
     throw new Exception("before");
   }