Add separate compilation test

And drop duplicate test into-inferred.scala

Author: odersky

tests/pos/into-separate/Test_2.scala

@@ -0,0 +1,16 @@
+//> using options -feature
+package test
+import language.experimental.into
+
+object Test:
+  given Conversion[Int, C] = C(_)
+
+  def f(x: T) = ()
+  f(1)      // ok
+
+  given stringToKeyword: Conversion[String, Keyword] = Keyword(_)
+
+  val dclKeywords = Set[Keyword]("def", "val") // ok
+  val keywords = dclKeywords + "if" + "then" + "else" // ok
+
+

tests/pos/into-separate/classes_1.scala

@@ -0,0 +1,8 @@
+package test
+
+import language.experimental.into
+
+into trait T
+class C(x: Int) extends T
+
+into class Keyword(str: String)