disable popen 'no duplicates' test for MinGW

This test, which seems quite complicated to fix, causes the MinGW build
to hang. Disable it for now, but, come back to it later.

* test-suite/tests/popen.test (mingw?): new constant
  (no duplicate): disable for mingw, for now

test-suite/tests/popen.test

@@ -1,6 +1,7 @@
 ;;;; popen.test --- exercise ice-9/popen.scm      -*- scheme -*-
 ;;;;
-;;;; Copyright 2003, 2006, 2010, 2011, 2013, 2014, 2020 Free Software Foundation, Inc.
+;;;; Copyright 2003, 2006, 2010, 2011, 2013, 2014, 2020
+;;;;           2021 Free Software Foundation, Inc.
 ;;;;
 ;;;; This library is free software; you can redistribute it and/or
 ;;;; modify it under the terms of the GNU Lesser General Public
@@ -22,6 +23,9 @@
   #:use-module (ice-9 rdelim))
 
 
+(define mingw?
+  (string-contains %host-type "-mingw32"))
+
 ;; read from PORT until eof is reached, return what's read as a string
 (define (read-string-to-eof port)
   (do ((lst '() (cons c lst))
@@ -104,6 +108,7 @@
    ;; sure that we are testing what the parent sees at a point where
    ;; the child has closed stdout but is still alive.
    (pass-if "no duplicate"
+     (when mingw? (throw 'unresolved))
      (let* ((c2p (pipe))
             (p2c (pipe))
             (port (with-error-to-port (cdr c2p)