Provide tests coverage

Problem/Motivation

Since this module is the basement for a number of other contrib projects, it is kind of critical to have tests coverage.

PS: We can tackle all tests in this issue, or use this as a [meta] issue to coordinate the effort while actioning the work in followups. For now work will be done in this issue until blockers are found.

Proposed resolution

Add test coverage for all features provided by Configuration Provider module.

Remaining tasks

Provide a test for:

• config.installer alter. Not sure what to test in here. Providers are not called, unless a module has got a "config/install" folder, since ConfigInstaller::installDefaultConfig() skips the block where getConfigToCreate() is called unless a "config/install" folder exists in the module being processed. Is this by design?
• config/install plugin logic
• config/optional plugin logic