test: add in a regression test for #7414

While the original reported issue does now compile, the pickling tests
fail with:

```diff
- .fun:((q: DepTest.Dep[DepTest.obj.type]#t.Dependent): scala.Unit)>@tests/pos/i7414.scala<223..235>
+ .fun:((q: DepTest.obj.Dependent): scala.Unit)>@tests/pos/i7414.scala<223..235>
```
This adds in a test for this, but adds it to the excludes. I'm assuming
this was what was meant by this comment: #7414 (comment).

Refs: #7414

Author: ckipp01

compiler/test/dotc/pos-test-recheck.excludes

@@ -18,3 +18,6 @@ gadt-cast-singleton.scala
 i11247.scala
 i6199b.scala
 t3440.scala
+
+# pickling related issue
+i7414.scala

tests/pos/i7414.scala

@@ -0,0 +1,12 @@
+// https://github.com/lampepfl/dotty/issues/7414
+
+object DepTest {
+  trait Trait {
+    case class Dependent()
+  }
+  object obj extends Trait
+  case class Dep[T <: Trait](t: T) {
+    def fun(q: t.Dependent): Unit = ???
+  }
+  Dep(obj).fun(obj.Dependent())
+}