Increase test coverage for `--sniffs` and `--exclude` options

[fredden]

Description

While working on https://github.com/PHPCSStandards/PHP_CodeSniffer/pull/344, it was noted that there are no existing tests for the code being changed. This pull request aims to add some tests to cover the existing behaviour.

Suggested changelog entry

• Improve test coverage of internal functionality

Related issues/external references

Related to #344

Types of changes

• Chore - no functional changes, only code coverage

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the Contribution Guidelines.
• [x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• [x] I have added tests to cover my changes.
• [x] I have verified that the code complies with the projects coding standards.

[fredden]

@jrfnl, I think this is ready for another review now. When we're happier with the final delta, I can rewrite the history so there are fewer commits before this is merged in.

[fredden]

:tada: Yes. A squash-merge via GitHub is fine with me. Thanks for your coaching here too @jrfnl.