patch 8.2.2632: not all command line arguments are tested

Problem:    Not all command line arguments are tested.
Solution:   Add tests for -D and -serverlist. (Dominique Pellé, closes #7992)
diff --git a/src/testdir/test_clientserver.vim b/src/testdir/test_clientserver.vim
index 6a73d93..2704fb6 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -43,6 +43,14 @@
   " When using valgrind it takes much longer.
   call WaitForAssert({-> assert_match(name, serverlist())})
 
+  if !has('win32')
+    if RunVim([], [], '--serverlist >Xtest_serverlist')
+      let lines = readfile('Xtest_serverlist')
+      call assert_true(index(lines, 'XVIMTEST') >= 0)
+    endif
+    call delete('Xtest_serverlist')
+  endif
+
   eval name->remote_foreground()
 
   call remote_send(name, ":let testvar = 'yes'\<CR>")