CA: Refactor invalid CSR tests into table-driven subtests. (#2868)
CA: Refactor invalid CSR tests into table-driven subtests. (#2868)

This will make it easier to add new tests of this form and will also
make it easier to adapt the tests to also test the precertificate +
certificate issuance flow.
jcjones/boulder,lucasallan/boulder,jfrazelle/boulder,lucasallan/boulder,jsha/boulder,letsencrypt/boulder,bmw/boulder,jcjones/boulder,letsencrypt/boulder,ibukanov/boulder,jcjones/boulder,jfrazelle/boulder,bmw/boulder,bmw/boulder,lucasallan/boulder,jfrazelle/boulder,jsha/boulder,jsha/boulder,ibukanov/boulder,jfrazelle/boulder,jcjones/boulder,ibukanov/boulder,letsencrypt/boulder