feat(openvpn): add a top-level test key with bootstrap time and handshake failure

[ainghazal]

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: spec(openvpn): ensure that the semantics of bootstrap_time are consistent between tunnel experiments probe#2758
• if you changed anything related to how experiments work and you need to reflect these changes in the ooni/spec repository, please link to the related ooni/spec pull request: modify openvpn nettest specification to conform to df-009-tunnel spec#295
• if you changed code inside an experiment, make sure you bump its version number

Description

Add bootstrap_time as a top-level test-key, making OpenVPN nettest conforming to df-009-tunnel. It also adds tunnel and failure keys with the semantics declared in that spec.

[DecFox]

LGTM!