* move `preflight` tests to `plugins` test folder
* move `container` tests to `plugins` test folder
* add a test that ensures that all built-in plugins have tests
This is based on a few assumptions:
1. All plugins exist in the `corePlugins.js` file and are exported as
`corePlugins`
2. Each exported plugin in that object has a file with a corresponding
name in the current (`tests/plugins/`) folder
* add dedicated plugin tests in the `plugins` folder
* add a test that ensures that all built-in variant plugins have tests
This is based on a few assumptions:
1. All variant plugins exist in the `corePlugins.js` file and are
exported as `variantPlugins`
2. Each exported plugin in that object has a file with a corresponding
name in the current (`tests/plugins/variants/`) folder
* add dedicated variant plugin tests in the `plugins/variants` folder
* cleanup basic usage test
* move existing animation tests to animation plugin tests
* move `apply-important-selector` tests to more appropriate location
