test: add coverage for unknown value to -blockfilterindex

[brunoerg]

This PR adds test coverage for the following init error:

bitcoin/src/init.cpp

Line 844 in 44037a2

return InitError(strprintf(_("Unknown -blockfilterindex value %s."), name));

Passing an unknown value to -blockfilterindex should throw an error.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #25193 (indexes: Read the locator's top block during init, allow interaction with reindex-chainstate by mzumsande)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[dunxen]

cr-ACK 295ff61