patch 8.2.0321: Vim9: ":execute" does not work yet

Problem:    Vim9: ":execute" does not work yet.
Solution:   Add ISN_EXECUTE. (closes #5699) Also make :echo work with more
            than one argument.

src/testdir/test_vim9_script.vim

@@ -1,6 +1,7 @@
 " Test various aspects of the Vim9 script language.
 
 source check.vim
+source view_util.vim
 
 " Check that "lines" inside ":def" results in an "error" message.
 func CheckDefFailure(lines, error)
@@ -692,5 +693,33 @@ def Test_substitute_cmd()
   delete('Xvim9lines')
 enddef
 
+def Test_execute_cmd()
+  new
+  setline(1, 'default')
+  execute 'call setline(1, "execute-string")'
+  assert_equal('execute-string', getline(1))
+  let cmd1 = 'call setline(1,'
+  let cmd2 = '"execute-var")'
+  execute cmd1 cmd2
+  assert_equal('execute-var', getline(1))
+  execute cmd1 cmd2 '|call setline(1, "execute-var-string")'
+  assert_equal('execute-var-string', getline(1))
+  let cmd_first = 'call '
+  let cmd_last = 'setline(1, "execute-var-var")'
+  execute cmd_first .. cmd_last
+  assert_equal('execute-var-var', getline(1))
+  bwipe!
+enddef
+
+def Test_echo_cmd()
+  echo 'something'
+  assert_match('^something$', Screenline(&lines))
+
+  let str1 = 'some'
+  let str2 = 'more'
+  echo str1 str2
+  assert_match('^some more$', Screenline(&lines))
+enddef
+
 
 " vim: ts=8 sw=2 sts=2 expandtab tw=80 fdm=marker