[chore] Update interactionPolicy sub-policy parsing in line with documented defaults (#4229)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Brings our parsing of unset sub-policies in line with the defaults documented here: https://docs.gotosocial.org/en/v0.19.1/federation/interaction_policy/#defaults-per-sub-policy

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4146

Part of https://codeberg.org/superseriousbusiness/gotosocial/issues/4026

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4229
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

testrig/teststructs.go

@@ -50,6 +50,7 @@ type TestStructs struct {
 	EmailSender         email.Sender
 	WebPushSender       *WebPushMockSender
 	TransportController transport.Controller
+	InteractionFilter   *interaction.Filter
 }
 
 func SetupTestStructs(
@@ -120,6 +121,7 @@ func SetupTestStructs(
 		EmailSender:         emailSender,
 		WebPushSender:       webPushSender,
 		TransportController: transportController,
+		InteractionFilter:   intFilter,
 	}
 }