Riseup tests improvements

[cyBerta]

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: RiseupVPN measurements frequently say "likely blocked" when server is down ooni/probe#1928 and riseupvpn: https://api.black.riseup.net:9001/json: connection_refused ooni/probe#1842
• 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: doc(riseupvpn): sync with ooniprobe 3.19 ooni/spec#273
• if you changed code inside an experiment, make sure you bump its version number

Description

This MR aims to improve the data quality of RiseupVPN tests by reduce the amount of false positives due to server side issues.
Major changes are:

• API failures are reported, but not necessarily a reason to stop the test early
• invalid CA's are indicated but not considered a failure and the tests will continue without TLS verification
• a blocked API will be indicated as well, but tried to be circumvented with a Tor+Snowflake tunnel.
• the API is only considered blocked if the main configuration end point is unreachable, RiseupVPN has multiple ways to get around other other API blockages or server side misbehavior (targets riseupvpn: https://api.black.riseup.net:9001/json: connection_refused ooni/probe#1842 (comment))
• overloaded gateways are not tested anymore
• only a subset of gateways are tested

[coveralls]

Pull Request Test Coverage Report for Build 4471791066

• 111 of 111 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 69.978%

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Totals
Change from base Build 4328715146:	0.1%
Covered Lines:	17139
Relevant Lines:	24492

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💛 - Coveralls

[cyBerta]

Another TODO:

• figure out if we can run obfs4 handshake tests
  EDIT: will be done in a separate MR