Add -source:3.8 option to all appropriate tests

Author: EugeneFlesselle

tests/neg/i3989e.scala

@@ -1,3 +1,5 @@
+//> using options -source:3.8
+
 object Test extends App {
   trait A[+X](val x: X)
   class B extends A(5) with A("hello") // error: A is extended twice // error: class B cannot be instantiated since it has conflicting base types Test.A[Int] and Test.A[String]

tests/pos/cb-companion-joins.scala

@@ -1,3 +1,4 @@
+//> using options -source:3.8
 import language.experimental.modularity
 
 trait M[Self]:

tests/pos/i10256.scala

@@ -1,3 +1,5 @@
+//> using options -source:3.8
+
 trait Foo[T <: Foo[T]] {
   type I <: Foo[I]
 }

tests/pos/i11064.scala

@@ -1,3 +1,5 @@
+//> using options -source:3.8
+
 trait TypedArray[T, Repr]
 
 trait Ops[T <: TypedArray[_, T]] {