Add return types to core tests code via Rector

Problem/Motivation

Adding return types to runtime code has BC concerns, but adding to test code should be straightforward and can be at least partially automated with rector.

Proposed resolution

Use rector with type coverage on core/tests.

Install rector

$ composer require --dev rector/rector

Configure rector.php file

<?php

declare(strict_types=1);

use Rector\TypeDeclaration\Rector\ClassMethod;
use Rector\TypeDeclaration\Rector\Class_\TypedPropertyFromCreateMockAssignRector;

return Rector\Config\RectorConfig::configure()
  ->withPaths([
    __DIR__ . '/core/tests',
  ])
  // First level before param and property types are affected.
  ->withTypeCoverageLevel(22)
  ->withSkip([
    // Skip as this adds unwanted phpdoc changes.
    ClassMethod\ReturnTypeFromStrictNewArrayRector::class,
    ClassMethod\ReturnTypeFromMockObjectRector::class,
    TypedPropertyFromCreateMockAssignRector::class,
  ])
  // Additional rules above level 22 that are useful
  ->withRules([
    ClassMethod\ReturnTypeFromStrictTypedCallRector::class,
    ClassMethod\ReturnUnionTypeRector::class,
    ClassMethod\ReturnTypeFromStrictParamRector::class,
    ClassMethod\ReturnTypeFromStrictFluentReturnRector::class,
    ClassMethod\ReturnNeverTypeRector::class,
  ]);

Run rector
$ ./vendor/bin/rector

Fix code style
$ composer phpcbf

Update baseline
$ ./vendor/bin/phpstan analyse -c core/phpstan.neon.dist --generate-baseline=core/.phpstan-baseline.php --memory-limit=2G

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet