[DX] Make the test suite work without credentials or external calls

[davidcornu]

Summary of the problem

Prior art

• [DX] Stub out remote calls in the test suite #10755
• [RSpec] Mock web requests (OpenAI, etc.) #8945
• Set up WebMock to catch network calls in tests #10829
• Integrate web mocking library #11061
• Add test coverage for OrganizerPositionInvitesController#create #11230 (which added WebMock)

Because our test suite makes external API calls and relies on certain credentials being available,

• It does not run successfully for third-party (including Dependabot) PRs as we don't provision all the environment variables
• It is a pain to get running locally as you need credentials for a bunch of services
• It does not run without a network connection

Describe your changes

I commented out my local .env file, configured WebMock to block all requests, and chipped away at all the errors until the whole thing ran successfully. This ended up involving:

1. Adding fallback values for TEST_URL_HOST (this seems like an odd thing to require in the first place)
2. Generating a fake Stripe API key when one isn't configured (only in the test env)
3. Generating an ephemeral encryption key for Lockbox if one isn't configured (only in the test env)
4. Generating ephemeral encryption keys for ActoveRecord::Encryption if they aren't configured (only in the test env)
5. Added a cached version of the EuCentralBank values for use in the test env
6. Added stubs and/or performed additional refactors to impacted tests

When (2), (3), and (4) take effect we log the ephemeral values to STDERR:

I've also removed WebMock.allow_net_connect! so any newly-introduced network calls will fail.

Notes

• Some of the changes in this PR were extracted into [Accounting] Clean up one-time jobs from transaction category backfill #11750
• This was purposefully opened from a fork to see if we can get a clean CI run from an outside contribution

[garyhtou]

this is very nice to have!

[garyhtou]

good catch