patch 8.2.2837: various code lines not covered by tests

Problem:    Various code lines not covered by tests.
Solution:   Add test cases. (Dominique Pellé, closes #8178)
diff --git a/src/testdir/test_startup.vim b/src/testdir/test_startup.vim
index 2d5fdcb..6e81470 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -622,6 +622,12 @@
   endfor
 
   if has('gui_gtk')
+    let out = split(system(GetVimCommand() .. ' --socketid'), "\n")
+    call assert_equal(1, v:shell_error)
+    call assert_match('^VIM - Vi IMproved .* (.*)$',          out[0])
+    call assert_equal('Argument missing after: "--socketid"', out[1])
+    call assert_equal('More info with: "vim -h"',             out[2])
+
     for opt in ['--socketid x', '--socketid 0xg']
       let out = split(system(GetVimCommand() .. ' ' .. opt), "\n")
       call assert_equal(1, v:shell_error)
@@ -629,6 +635,7 @@
       call assert_equal('Invalid argument for: "--socketid"', out[1])
       call assert_equal('More info with: "vim -h"',           out[2])
     endfor
+
   endif
 endfunc