Add test filter parameter to ios_unit_test

[dostrander]

This adds the ability to add your test_filter directly to the args passed into ios_unit_test, allowing you to specify a test filter on rule creation for your target.

This was talked about in this discussion and helps provide a way to shard targets.

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[dostrander]

Something is wrong with the logic of the ios_test_runner template shell script. Looking into it now

[keith]

Thanks!