Update checkfiles

Author: aherlihy

tests/neg/classOf.check

@@ -2,13 +2,19 @@
 6 |  def f1[T] = classOf[T] // error
   |                      ^
   |                      T is not a class type
+  |
+  | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg/classOf.scala:7:32 ---------------------------------------------------------------------
 7 |  def f2[T <: String] = classOf[T] // error
   |                                ^
   |                                T is not a class type
   |
   |                                where:    T is a type in method f2 with bounds <: String
+  |
+  | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg/classOf.scala:9:18 ---------------------------------------------------------------------
 9 |  val y = classOf[C { type I = String }] // error
   |                  ^^^^^^^^^^^^^^^^^^^^^
   |                  Test.C{type I = String} is not a class type
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i13808.check

@@ -2,7 +2,11 @@
 13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
    |                                     ^^^^^^^^^^
    |                                     OpaqueTypes.OpaqueType is not a class type
+   |
+   | longer explanation available when compiling with `-explain`
 -- [E170] Type Error: tests/neg/i13808.scala:13:49 ---------------------------------------------------------------------
 13 |case class Boom[A](value: A) derives OpaqueType, Foo // error // error
    |                                                 ^^^
    |                                                 FooModule.Foo is not a class type
+   |
+   | longer explanation available when compiling with `-explain`