add testcase for #6060 (#15366)

* add testcase for #6060

* Remove overspecific testament spec

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/generics/t6060.nim

@@ -0,0 +1,11 @@
+import tables
+
+type MyTab[A,B] = distinct TableRef[A,B]
+
+proc `$`[A,B](t: MyTab[A,B]): string =
+  "My special table " & $TableRef[A,B](t)
+
+proc create[A,B](): MyTab[A,B] = MyTab(newTable[A,B]())
+
+var a = create[int,int]()
+doAssert $a == "My special table {:}"