patch 8.0.0475: not enough testing for the client-server feature

Problem:    Not enough testing for the client-server feature.
Solution:   Add more tests.  Add the remote_startserver() function.  Fix that
            a locally evaluated expression uses function-local variables.
diff --git a/src/testdir/test_clientserver.vim b/src/testdir/test_clientserver.vim
index 55cc98e..55f440f 100644
--- a/src/testdir/test_clientserver.vim
+++ b/src/testdir/test_clientserver.vim
@@ -30,6 +30,31 @@
   call WaitFor('remote_expr("' . name . '", "testvar") == "yes"')
   call assert_equal('yes', remote_expr(name, "testvar"))
 
+  if has('unix') && has('gui') && !has('gui_running')
+    " Running in a terminal and the GUI is avaiable: Tell the server to open
+    " the GUI and check that the remote command still works.
+    " Need to wait for the GUI to start up, otherwise the send hangs in trying
+    " to send to the terminal window.
+    call remote_send(name, ":gui -f\<CR>")
+    sleep 500m
+    call remote_send(name, ":let testvar = 'maybe'\<CR>")
+    call WaitFor('remote_expr("' . name . '", "testvar") == "maybe"')
+    call assert_equal('maybe', remote_expr(name, "testvar"))
+  endif
+
+  call assert_fails('call remote_send("XXX", ":let testvar = ''yes''\<CR>")', 'E241')
+
+  " Expression evaluated locally.
+  if v:servername == ''
+    call remote_startserver('MYSELF')
+    call assert_equal('MYSELF', v:servername)
+  endif
+  let g:testvar = 'myself'
+  call assert_equal('myself', remote_expr(v:servername, 'testvar'))
+
+  call remote_send(name, ":call server2client(expand('<client>'), 'got it')\<CR>", 'g:myserverid')
+  call assert_equal('got it', remote_read(g:myserverid))
+
   call remote_send(name, ":qa!\<CR>")
   call WaitFor('job_status(g:job) == "dead"')
   if job_status(g:job) != 'dead'