test: add in regression test for scala#7445 (scala#17475)

So this adds in a test that shows that the minimized version of the
original issue is fixed, but the original report now fails the pickling
tests. I've gone ahead and added a test for that as well, but added it
to the excludes. I'll reopen the issue and mention this update.

Author: nicolasstucki

compiler/test/dotc/pos-test-pickling.blacklist

@@ -95,3 +95,6 @@ i4176-gadt.scala
 i13974a.scala
 
 java-inherited-type1
+
+# recursion limit exceeded
+i7445b.scala

tests/pos/i7445a.scala

@@ -0,0 +1,10 @@
+// https://github.com/lampepfl/dotty/issues/7445
+
+object Main {
+  type O1[A] = {
+    type OutInner[X] = Unit
+    type Out = OutInner[A]
+  }
+
+  def f1: O1[Int]#Out = ???
+}

tests/pos/i7445b.scala

@@ -0,0 +1,12 @@
+// https://github.com/lampepfl/dotty/issues/7445
+
+type O1[A] = {
+  type OutInner[Ts] <: Tuple = Ts match {
+    case EmptyTuple => EmptyTuple
+    case h *: t => h *: OutInner[t]
+  }
+
+  type Out = OutInner[A]
+}
+
+def f1: O1[(Int, Int)]#Out = ???