patch 8.0.0474: the client-server feature is not tested

Problem:    The client-server feature is not tested.
Solution:   Add a test.

src/testdir/test_clientserver.vim

@@ -0,0 +1,42 @@
+" Tests for the +clientserver feature.
+
+if !has('job') || !has('clientserver')
+  finish
+endif
+
+source shared.vim
+
+func Test_client_server()
+  let cmd = GetVimCommand()
+  if cmd == ''
+    return
+  endif
+  let name = 'XVIMTEXT'
+  let cmd .= ' --servername ' . name
+  let g:job = job_start(cmd, {'stoponexit': 'kill', 'out_io': 'null'})
+  call WaitFor('job_status(g:job) == "run"')
+  if job_status(g:job) != 'run'
+    call assert_true(0, 'Cannot run the Vim server')
+    return
+  endif
+
+  " Takes a short while for the server to be active.
+  call WaitFor('serverlist() =~ "' . name . '"')
+  call assert_match(name, serverlist())
+
+  call remote_foreground(name)
+
+  call remote_send(name, ":let testvar = 'yes'\<CR>")
+  call WaitFor('remote_expr("' . name . '", "testvar") == "yes"')
+  call assert_equal('yes', remote_expr(name, "testvar"))
+
+  call remote_send(name, ":qa!\<CR>")
+  call WaitFor('job_status(g:job) == "dead"')
+  if job_status(g:job) != 'dead'
+    call assert_true(0, 'Server did not exit')
+    call job_stop(g:job, 'kill')
+  endif
+endfunc
+
+" Uncomment this line to get a debugging log
+" call ch_logfile('channellog', 'w')