Parse unittest attributes and write them to the publictest tester

[wilzbach]

While the underlying issue is https://issues.dlang.org/show_bug.cgi?id=19873, fixing this allows the Phobos PR to move on for now.

tl;dr: the publictest extractor now extracts the respective system attributes too.

Required for dlang/phobos#6931

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + tools#369"

[wilzbach]

(passing on TravisCI now - AppVeyor failure is unrelated.)

[PetarKirov]

I don't think this would work with attributes whose syntax doesn't start with @ , like pure and nothrow

[wilzbach]

Yup, but DScanner treats these guys differently:

$ dscanner --ast foo.d | xmllint --format -
<?xml version="1.0"?>
<module>
  <declaration>
    <attribute attribute="pure"/>
    <attribute>
      <atAttribute>
        <identifier>safe</identifier>
      </atAttribute>
    </attribute>
    <unittest>
      <blockStatement>
</blockStatement>
    </unittest>
  </declaration>
</module>

[wilzbach]

But yeah it's probably a good idea to extract these guys too while we're at it: #371