tests: os: address race in internet con. sharing tests

[rcooke-warwick]

For some device types it's been noted that it takes longer than the time it takes do do one check of the iptables rules for the intended rules to appear, leading to a failure. This aims to make the check more consistent by checking 5 times before failing.

Change-type: patch

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[rcooke-warwick]

@resin-jenkins retest this please

[acostach]

@resin-jenkins retest this please

[rcooke-warwick]

@resin-jenkins retest this please

[rcooke-warwick]

@resin-jenkins retest this please