posix_spawn_file_actions_addclose tests: Avoid test failure on Cygwin

[Bruno Haible]

If Gnulib were not to override Cygwin's posix_spawn implementation, two
tests would fail:

FAIL: test-posix_spawn_file_actions_addclose
============================================

../../gltests/test-posix_spawn_file_actions_addclose.c:63: assertion 
'posix_spawn_file_actions_addclose (&actions, bad_fd) == EBADF' failed
FAIL test-posix_spawn_file_actions_addclose.exe (exit status: 134)

FAIL: test-posix_spawnp-script
==============================

subprocess terminated with unexpected exit status 0
FAIL test-posix_spawnp-script.exe (exit status: 1)

The first of these failures is a problem in Gnulib's test. Fixed like this:


2023-04-16  Bruno Haible  <br...@clisp.org>

        posix_spawn_file_actions_addclose tests: Avoid test failure on Cygwin.
        * tests/test-posix_spawn_file_actions_addclose.c (main): Skip an
        unportable test also on Cygwin.

diff --git a/tests/test-posix_spawn_file_actions_addclose.c 
b/tests/test-posix_spawn_file_actions_addclose.c
index f900415dae..f28017e585 100644
--- a/tests/test-posix_spawn_file_actions_addclose.c
+++ b/tests/test-posix_spawn_file_actions_addclose.c
@@ -55,8 +55,8 @@ main (void)
     ASSERT (posix_spawn_file_actions_addclose (&actions, -1) == EBADF);
   }
   /* This behaviour is not mandated by POSIX, but happens to pass on all
-     platforms except musl libc.  */
-#if !defined MUSL_LIBC
+     platforms except musl libc and Cygwin.  */
+#if !(defined MUSL_LIBC || defined __CYGWIN__)
   {
     int bad_fd = big_fd ();
     errno = 0;