Add a test for JSON schema to check that all mutators are indeed validated.

[maks-rafalko]

I wonder if we should have a test for this list, to check that all mutators are indeed present.

Originally posted by @sanmai in #782 (comment)

I just realized that we will definitely forget updating json schema after adding new mutators, which I've already done twice :D (#787, #784). AutoReview test is a must have here

[maks-rafalko]

It exists and works as expected

infection/tests/phpunit/Configuration/Schema/SchemaConfigurationFactoryTest.php

Lines 1817 to 1828 in 63046b6

	foreach ($genericMutatorNamesList as $mutator) {
	yield '[mutators][generic][' . $mutator . '] enabled' => [
	<<<JSON
	{
	"source": {
	"directories": ["src"]
	},
	"mutators": {
	"$mutator": true
	}
	}
	JSON