add tests/stdlib/tsugar.nim ; add test case from @mratsim

Author: timotheecour

lib/pure/sugar.nim

@@ -241,4 +241,4 @@ proc distinctBase*[T](a: T, recursive: static[bool] = false): auto =
 
 when isMainModule:
   # pending https://github.com/nim-lang/Nim/issues/7280
-  discard distinctBase[int](0)
+  discard distinctBase[int](0, recursive = true)

tests/stdlib/tsugar.nim

@@ -0,0 +1,11 @@
+discard """
+  file: "tsugar.nim"
+  output: ""
+"""
+import sugar
+
+block distinctBase:
+  type
+    Foo[T] = distinct seq[T]
+  var a: Foo[int]
+  doAssert a.type.distinctBase is seq[int]