bug#71980: [3.0.10] posix.test uses depcrecated tmpnam

With `--disable-tmpnam'  argument to configure, posix.test fails: 
missing `tmpnam`.

patch attached

--- test-suite/tests/posix.test-orig    2024-07-03 15:45:16.352132881 -0700
+++ test-suite/tests/posix.test    2024-07-07 09:21:53.342855356 -0700
@@ -361,20 +361,21 @@
        (status:exit-val (system* "something-that-does-not-exist"))))

    (pass-if-equal "https://bugs.gnu.org/52835"
-      "bong\n"
-    (let ((file (tmpnam)))
+    "bong\n"
+    (let ((port (mkstemp "T-XXXXXX")))
        ;; Redirect stdout and stderr to FILE.
        (define status
-        (call-with-output-file file
-          (lambda (port)
-            (with-output-to-port port
+        (with-output-to-port port
+          (lambda ()
+            (with-error-to-port port
                (lambda ()
-                (with-error-to-port port
-                  (lambda ()
-                    (system* "sh" "-c" "echo bong >&2"))))))))
+                (system* "sh" "-c" "echo bong >&2"))))))

        (and (zero? (status:exit-val status))
-           (call-with-input-file file get-string-all))))
+           (zero? (seek port 0 SEEK_SET))
+           (let ((contents (get-string-all port)))
+             (close-port port)
+             contents))))

    (pass-if-equal "https://bugs.gnu.org/63024"
        0

--- test-suite/tests/posix.test-orig	2024-07-03 15:45:16.352132881 -0700
+++ test-suite/tests/posix.test	2024-07-07 09:21:53.342855356 -0700
@@ -361,20 +361,21 @@
       (status:exit-val (system* "something-that-does-not-exist"))))
 
   (pass-if-equal "https://bugs.gnu.org/52835"
-      "bong\n"
-    (let ((file (tmpnam)))
+    "bong\n"
+    (let ((port (mkstemp "T-XXXXXX")))
       ;; Redirect stdout and stderr to FILE.
       (define status
-        (call-with-output-file file
-          (lambda (port)
-            (with-output-to-port port
+        (with-output-to-port port
+          (lambda ()
+            (with-error-to-port port
               (lambda ()
-                (with-error-to-port port
-                  (lambda ()
-                    (system* "sh" "-c" "echo bong >&2"))))))))
+                (system* "sh" "-c" "echo bong >&2"))))))
 
       (and (zero? (status:exit-val status))
-           (call-with-input-file file get-string-all))))
+           (zero? (seek port 0 SEEK_SET))
+           (let ((contents (get-string-all port)))
+             (close-port port)
+             contents))))
 
   (pass-if-equal "https://bugs.gnu.org/63024"
       0