tests: Make sure child process of 'pivot-root' test exits.

* tests/syscalls.scm ("pivot-root"): Use 'test-equal'.  Wrap child body
in 'dynamic-wind'.

tests/syscalls.scm

@@ -148,11 +148,15 @@
 
 (unless perform-container-tests?
   (test-skip 1))
-(test-assert "pivot-root"
+(test-equal "pivot-root"
+  #t
   (match (pipe)
     ((in . out)
      (match (clone (logior CLONE_NEWUSER CLONE_NEWNS SIGCHLD))
        (0
+        (dynamic-wind
+          (const #t)
+          (lambda ()
             (close in)
             (call-with-temporary-directory
              (lambda (root)
@@ -165,8 +169,9 @@
                  (pivot-root root put-old)
                  ;; The test file should now be located inside the root directory.
                  (write (file-exists? "/test") out)
-             (close out))))
-        (primitive-exit 0))
+                 (close out)))))
+          (lambda ()
+            (primitive-exit 0))))
        (pid
         (close out)
         (let ((result (read in)))