Add convenient test assertions for web APIs

[dunglas]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	n/a
License	MIT
Doc PR	api-platform/docs#875

This is a follow up PR to #2608. It introduces convenient PHPUnit assertions to test web APIs. A quick example is worth thousand words:

<?php

use use ApiPlatform\Core\Bridge\Symfony\Bundle\Test\ApiTestCase;

class ClientTest extends ApiTestCase
{
    public function testMyApi(): void
    {
        self::createClient()->request('GET', '/secured_dummies', ['auth_basic' => ['dunglas', 'kevin']]);

        $this->assertResponseIsSuccessful();
        $this->assertResponseHeaderSame('content-type', 'application/ld+json; charset=utf-8');
        // All web assertions provided by Symfony can also be used

        // Exact matching
        $this->assertJsonEquals([
            '@context' => '/contexts/SecuredDummy',
            '@id' => '/secured_dummies',
            '@type' => 'hydra:Collection',
            'hydra:member' => [],
            'hydra:totalItems' => 0,
        ]);

        // Partial matching
        $this->assertJsonContains(
            [
                '@context' => '/contexts/SecuredDummy',
                '@id' => '/secured_dummies',
            ]
        );

        // The JSON Schema can be represented as a string or a PHP array
        $this->assertMatchesJsonSchema(<<<JSON
{
  "type": "object",
  "properties": {
    "@context": {"pattern": "^/contexts/SecuredDummy$"},
    "@id": {"pattern": "^/secured_dummies$"},
    "@type": {"pattern": "^hydra:Collection"},
    "hydra:member": {}
  },
  "additionalProperties": true,
  "required": ["@context", "@id", "@type", "hydra:member"]
}
JSON
        );
    }
}

All HTTP assertions provided by Symfony can be used!

This PR also migrates our test suite to PHPUnit 8. Related to symfony/symfony#32207.