Fix test

The test previously compiled by accident due to the "more implicit parameters win" rule.
Once that rule got changed it should have failed, but it seems the error (illegal
forward reference) was not detected. Rebasing this commit on latest master detects
the error:
```
  implicit val co_i: Conversion[Char, Position[CharSequence]] = the[Conversion[Char, Position[CharSequence]]]
                                                                                                             ^
                                             co_i is a forward reference extending over the definition of co_i
```
The fix to the test avoids the error again.

Author: odersky

tests/pos/i5978.scala

@@ -63,14 +63,18 @@ package p3 {
       import implied TextParser._
       import TextParser._
 
-      val tp_v: TokenParser[Char, Position[CharSequence]] = TextParser.TP
-      val tp_i = the[TokenParser[Char, Position[CharSequence]]]
-      implicit val co_i: Conversion[Char, Position[CharSequence]] = the[Conversion[Char, Position[CharSequence]]]
-      val co_x : Position[CharSequence] = 'x'
+      val co_i: Conversion[Char, Position[CharSequence]] = the[Conversion[Char, Position[CharSequence]]]
 
       {
-        implied XXX for Conversion[Char, Position[CharSequence]] = co_i
-        val co_y : Position[CharSequence] = 'x'
+        val tp_v: TokenParser[Char, Position[CharSequence]] = TextParser.TP
+        val tp_i = the[TokenParser[Char, Position[CharSequence]]]
+        implied for Conversion[Char, Position[CharSequence]] = co_i
+        val co_x : Position[CharSequence] = 'x'
+
+        {
+          implied XXX for Conversion[Char, Position[CharSequence]] = co_i
+          val co_y : Position[CharSequence] = 'x'
+        }
       }
     }
   }