patch 8.2.4395: some code lines not covered by tests

Problem:    Some code lines not covered by tests.
Solution:   Add a few more test cases.  Fix getting more than one error for
            invalid assignment.
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index ecc9c64..65d6dc8 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -3762,7 +3762,15 @@
   v9.CheckScriptFailure(lines, 'E476:')
 enddef
 
+" The following messes up syntax highlight, keep near the end.
 if has('python3')
+  def Test_python3_command()
+    py3 import vim
+    py3 vim.command("let g:done = 'yes'")
+    assert_equal('yes', g:done)
+    unlet g:done
+  enddef
+
   def Test_python3_heredoc()
     py3 << trim EOF
       import vim
@@ -3778,7 +3786,6 @@
   enddef
 endif
 
-" This messes up syntax highlight, keep near the end.
 if has('lua')
   def Test_lua_heredoc()
     g:d = {}