mcastmngr: fix mcast stats script

Fixed worng temp_igmp_file variable accessing, which caused the same 'temp_igmp_file' file to be used each time 'ubus call mcast stats' was called instead of /tmp/igmp_stats_ file. Because of this, temp file was sometimes overwritten by the next call before it had time to read and the IPTV_IGMP_4-12-1-24_2401.py test ended with an error.

A waiting loop has also been added so that mcproxy has time to process the signal and write the dump to a file.