Ensure all built-in plugins have tests (#11618)

* 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

Author: RobinMalfait

tests/util/apply-important-selector.test.js renamed to tests/apply-important-selector.test.js

@@ -1,4 +1,4 @@
-import { applyImportantSelector } from '../../src/util/applyImportantSelector'
+import { applyImportantSelector } from '../src/util/applyImportantSelector'
 
 it.each`
   before                                        | after