test: add in a regression test for scala#14278 (scala#17510)

[skip community_build]

closes scala#14278

Author: nicolasstucki

tests/pos/i14278.scala

@@ -0,0 +1,6 @@
+// https://github.com/lampepfl/dotty/issues/14278
+class Foo
+
+extension (foo: Foo)
+  def patch(arg: List[Int], arg2: Int = 0): Unit = {}
+  def patch(arg: Int): Unit = patch(List(arg))