asn1: add extra test case for optional RawValue
asn1: add extra test case for optional RawValue

The preceding commit replaces our earlier commit 86236dd4f3dfi
which added support for parsing fields with an optional raw value.

Extend the test case to confirm that the new code works in all cases.

RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go,RJPercival/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go,daviddrysdale/certificate-transparency-go,google/certificate-transparency-go