feat: add fallback domain names for openvpn experiment

[ainghazal]

while working on this, I also gave more priority to possible oonirun descriptors passed in the command line.

• Related: #2805

Checklist

• I have read the contribution guidelines
• reference issue for this pull request: add fallback domains for openvpn test probe#2805
• 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:
• if you changed code inside an experiment, make sure you bump its version number

Description

Add fallback domains for openvpn default endpoints to be probed.

[ainghazal]

changed the logic after 1:1 with @hellais, ready for review

[ainghazal]

To clarify, I've added a few more comments to the docstring of the Load() function. An error is ignored (logged now); returning the empty list (in case of all bogons) is equivalent of passing an empty input list to the experiment.

@hellais: here I've pointed both domains to localhost in /etc/hosts, see:

[      0.328261] <info> Looking up your location; please be patient...
[      0.328277] <info> iplookup: using cloudflare
[      0.328407] <info> sessionresolver: lookup www.cloudflare.com using https://wikimedia-dns.org/dns-query... started
[      0.376407] <info> sessionresolver: lookup www.cloudflare.com using https://wikimedia-dns.org/dns-query... ok
[      0.485202] <info> - country: NO
[      0.485209] <info> - network: FOOBAR (AS0)
[      0.485213] <info> - resolver's IP: 172.253.5.153
[      0.485216] <info> - resolver's network: Google LLC (AS15169)
[      0.485338] <warn> Picking from default OpenVPN endpoints
[      0.485343] <warn> No targets loaded from default endpoints
[      0.485351] <info> experiment: recv   0.00  byte, sent   0.00  byte
[      0.485519] <info> whole session: recv   3.86 kbyte, sent   1.46 kbyte

To trace the execution path, and reason about it, I think the right place to go is the the call to newInputProcessor, which passes a targetList (the targetList this experiment provides, in this case).

In turn, inputprocessor.Run calls inputprocessor.run, which loops over the array of inputs. I think this ensures that an empty array runs the experiment zero times.