Add test for infix operator usage from Scala 2 libraries

Fixes #7304

Author: tgodzik

tests/pos/i7304.scala

@@ -0,0 +1,9 @@
+@main def test =
+  val myregex_r = "\\s+".r
+  val text = "adggfgf dfg"
+  myregex_r findFirstMatchIn text
+  text takeRight 5
+  val func = (a: Int) => a + 1
+  List(1,2, 3) map func
+  text stripPrefix "adgg"
+