patch 8.2.1100: Vim9: cannot use line break in :execute argument

Problem:    Vim9: cannot use line break in :execute, :echomsg and :echoerr
            argument.
Solution:   Check for line break.

src/testdir/test_vim9_script.vim

@@ -1294,6 +1294,19 @@ def Test_execute_cmd()
   call CheckDefFailure(['execute "cmd"# comment'], 'E488:')
 enddef
 
+def Test_execute_cmd_vimscript()
+  " only checks line continuation
+  let lines =<< trim END
+      vim9script
+      execute 'g:someVar'
+                .. ' = ' ..
+                   '28'
+      assert_equal(28, g:someVar)
+      unlet g:someVar
+  END
+  CheckScriptSuccess(lines)
+enddef
+
 def Test_echo_cmd()
   echo 'some' # comment
   echon 'thing'
@@ -1321,6 +1334,18 @@ def Test_echomsg_cmd()
   call CheckDefFailure(['echomsg "xxx"# comment'], 'E488:')
 enddef
 
+def Test_echomsg_cmd_vimscript()
+  " only checks line continuation
+  let lines =<< trim END
+      vim9script
+      echomsg 'here'
+                .. ' is ' ..
+                   'a message'
+      assert_match('^here is a message$', Screenline(&lines))
+  END
+  CheckScriptSuccess(lines)
+enddef
+
 def Test_echoerr_cmd()
   try
     echoerr 'something' 'wrong' # comment
@@ -1329,6 +1354,21 @@ def Test_echoerr_cmd()
   endtry
 enddef
 
+def Test_echoerr_cmd_vimscript()
+  " only checks line continuation
+  let lines =<< trim END
+      vim9script
+      try
+        echoerr 'this'
+                .. ' is ' ..
+                   'wrong'
+      catch
+        assert_match('this is wrong', v:exception)
+      endtry
+  END
+  CheckScriptSuccess(lines)
+enddef
+
 def Test_for_outside_of_function()
   let lines =<< trim END
     vim9script