patch 8.0.1539: no test for the popup menu positioning

Problem:    No test for the popup menu positioning.
Solution:   Add a screendump test for the popup menu.
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index ca87ccf..e49c8ed 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -5,9 +5,7 @@
 endif
 
 source view_util.vim
-if has('terminal')
-  source screendump.vim
-endif
+source screendump.vim
 
 func GetSyntaxItem(pat)
   let c = ''
@@ -528,10 +526,7 @@
 
 " Check highlighting for a small piece of C code with a screen dump.
 func Test_syntax_c()
-  " Need to be able to run terminal Vim with 256 colors.
-  " On MS-Windows the console only has 16 colors and the GUI can't run in a
-  " terminal.
-  if !has('terminal') || has('win32')
+  if !CanRunVimInTerminal()
     return
   endif
   call writefile([
@@ -561,7 +556,7 @@
   let $COLORFGBG = '15;0'
 
   let buf = RunVimInTerminal('Xtest.c', {})
-  call VerifyScreenDump(buf, 'Test_syntax_c_01')
+  call VerifyScreenDump(buf, 'Test_syntax_c_01', {})
   call StopVimInTerminal(buf)
 
   let $COLORFGBG = ''