add forwardCompat test for export in extension

Author: bishabosha

tests/pos/forwardCompat-exportInExtension/Lib_1_r3.0.scala

@@ -0,0 +1,13 @@
+case class ShortString(value: String)
+
+class StringOps(x: ShortString):
+  def *(n: Int): ShortString = ???
+  def capitalize: ShortString = ???
+
+object stringsyntax:
+
+  extension (x: ShortString)
+    def take(n: Int): ShortString = ???
+    def drop(n: Int): ShortString = ???
+    private def moreOps = StringOps(x)
+    export moreOps.*

tests/pos/forwardCompat-exportInExtension/Test_2_c3.0.2.scala

@@ -0,0 +1,7 @@
+import stringsyntax.*
+
+def test =
+  ShortString("Hello").take(2)
+  ShortString("Hello").drop(3)
+  ShortString("Hello") * 5
+  ShortString("Hello").capitalize