Add return types to test module code

Problem/Motivation

There are currently 7866 missing return types in the phpstan baseline.

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

See \Rector\Config\Level\TypeDeclarationLevel::RULES for a list of rules. The array index correlates to the level, so 22 gives us all the way up to ReturnNullableTypeRector::class. This is the first level before rector starts to modify param types, which is out of scope here. There are a few other exceptions/additions documented in the proposed resolution.

This removes 409 of the errors, and reduces the baseline by 2454 lines.

Steps to reproduce

$ grep "missingType.return" core/.phpstan-baseline.php | wc -l
7866

Proposed resolution

Use rector with type coverage on core/modules/*/tests/modules.

Install rector

$ composer require --dev rector/rector

Configure rector.php file

<?php
use Rector\TypeDeclaration\Rector\ClassMethod;
return Rector\Config\RectorConfig::configure()
  ->withPaths([
    __DIR__ . '/core/modules/*/tests/modules',
    __DIR__ . '/core/modules/config/tests/config_*',
    __DIR__ . '/core/modules/dynamic_page_cache/tests/dynamic_page_cache_test',
    __DIR__ . '/core/modules/file/tests/file_*',
    __DIR__ . '/core/modules/filter/tests/filter_*',
    __DIR__ . '/core/modules/language/tests/language_*',
    __DIR__ . '/core/modules/language/tests/test_module',
    __DIR__ . '/core/modules/menu_link_content/tests/menu_*',
    __DIR__ . '/core/modules/menu_link_content/tests/outbound_*',
    __DIR__ . '/core/modules/navigation/tests/navigation_*',
    __DIR__ . '/core/modules/node/tests/node_*',
    __DIR__ . '/core/modules/options/tests/options_*',
    __DIR__ . '/core/modules/serialization/tests/serialization_test',
  ])
  // First level before param and property types are affected.
  ->withTypeCoverageLevel(22)
  ->withSkip([
    // Skip as this adds unwanted phpdoc changes.
    ClassMethod\ReturnTypeFromStrictNewArrayRector::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