Fix status code when bad command name is entered

This commit fixes a bug which causes that

   fish -c ')'; echo $status

("Illegal command name" error) returns 0. This is inconsistent with
e.g. when trying to run non-existent command:

   fish -c 'invalid-command'; echo $status

("Unknown command" error) which correctly returns 127.

A new status code,

    STATUS_ILLEGAL_CMD = 123

is introduced - which is returned whenever the 'Illegal command name *'
message is printed.

This commit also adds a test which checks if valid commands return 0,
while commands with illegal name return status code 123.

Fixes #3606.

src/parse_execution.cpp

@@ -832,6 +832,7 @@ parse_execution_result_t parse_execution_context_t::populate_plain_process(
     bool expanded = expand_one(cmd, EXPAND_SKIP_CMDSUBST | EXPAND_SKIP_VARIABLES, NULL);
     if (!expanded) {
         report_error(statement, ILLEGAL_CMD_ERR_MSG, cmd.c_str());
+        proc_set_last_status(STATUS_ILLEGAL_CMD);
         return parse_execution_errored;
     }