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

[hellais]

Will this result in a hard failure of the test? Is there some way we can ensure that's not the case? For example we might want for certain runs in particular countries to not run the vpn tests and it would be ideal if that results in a noop as opposed to the test being considered a failure.

[ainghazal]

if this function returns error, we simply return an empty targets list, see:

probe-cli/internal/experiment/openvpn/richerinput.go

Line 113 in d9c4b3e

if inputs, err := pickOONIOpenVPNTargets(addrs); err == nil {

[ainghazal]

see the comment below

[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.