Nyall Dawson d4ad063f45 Allow specifying a 'primary key' field when comparing layers for
processing tests

Some algorithms will return results in different orders, e.g.
due to the use of dicts or other methods which do not guarantee
a fixed return order.

Using a primary key to do the feature match allows us to flexibly
handle these situations and provide tests for these algorithms.
2017-08-05 22:10:18 +10:00
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