src/t: Fix dpkg-divert.t when skipping test due to missing /dev/full

We need to restore the admindir permissions or the subsequent
non-skipped test will not pass.

src/t/dpkg_divert.t

@@ -620,10 +620,12 @@ SKIP: {
     system("chmod 500 $admindir");
     call_divert(["$testdir/foo"], expect_failure => 1, expect_stderr_like => qr/create.*new/);
 
+    system("chmod 755 $admindir");
+
     SKIP: {
         skip 'device /dev/full is not available', 2 if not -c '/dev/full';
 
-        system("chmod 755 $admindir; ln -s /dev/full $admindir/diversions-new");
+        system("ln -s /dev/full $admindir/diversions-new");
         call_divert(["$testdir/foo"], expect_failure => 1,
                     expect_stderr_like => qr/(write|flush|close).*new/);
     }