Test boot_partition when fw_setenv fails (non-zero exit code).

-- File fs/open~_tmp_sysinfo_board_name.txt --
linksys,e7350
-- End --

-- File fs/popen~fw_printenv_-n_bootimage_2_dev_null.txt --
1
-- End --

-- File fs/popen~fw_setenv_bootimage_2_2_dev_null_echo_.txt --
1
-- End --

-- Testcase --
let ar = require('advanced_reboot');
let api = ar["luci.advanced-reboot"];
let result = api.boot_partition.call({ args: { number: "2" } });
printf("error: %s\n", result.error);
printf("args: %J\n", result.args);
-- End --

-- Expect stdout --
error: ERR_SET_ENV
args: [ "bootimage", "2" ]
-- End --