Test of `WP_CLI_CONFIG_SPEC_FILTER_CALLBACK` should be in `flags.feature` file

The test for the `WP_CLI_CONFIG_SPEC_FILTER_CALLBACK` constant seems to be in the wrong file. It is in the `config.feature` file (which deals with the `wp-cli.yml` et al.), but it should probably rather be in the `flags.feature` file.

_Originally posted by @schlessera in https://github.com/wp-cli/wp-cli/issues/5664#issuecomment-1275559869_
      