[RISCV][test] Expand test coverage of RISCVISAInfo

D144343 added C++ unit tests for the newly added
RISCVISAInfo::parseArchStringNormalized. This patch adds test coverage
for parseArchString and the toFeatureVector member method.

Posting for review as it would be useful if someone can take a quick
look to check they don't feel I've missed testing an important aspect of
the current behaviour.

Differential Revision: https://reviews.llvm.org/D145879