arch-test clone and maintenance subcommands

[neelgala]

since the framework and the tests are now in separate repos, it seems like a convenient feature to have riscof clone the correct repository from github. This way, a user simply needs to install riscof and run a subcommand to clone the correct version of the riscv-arch-test repo.

This should be implemented as a separate subcommand (maybe riscof test-clone). This sub-command shuold be able to :

• clone the riscv-arch-test repo and checkout the latest tag
• clone the riscv-arch-test repo and cehckout a specif version by tag or commit
• update an existing cloned repo to a specific tag/version

Some enterprise systems/environment may typically disable/restrict remote access or git commands, and thus this command should be completely optional and should not have any dependencies on the rest of the framework. This should be treated as an optional feature.