test(Misc): Add test coverage to the JsonPathReplacer

tests/src/Unit/JsonPathReplacerTest.php

+<?php
+
+namespace Drupal\Tests\subrequests\Unit;
+
+use Drupal\subrequests\JsonPathReplacer;
+use Drupal\subrequests\Subrequest;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\HttpFoundation\Response;
+
+/**
+ * @coversDefaultClass \Drupal\subrequests\JsonPathReplacer
+ * @group subrequests
+ */
+class JsonPathReplacerTest extends UnitTestCase {
+
+  /**
+   * @var \Drupal\subrequests\JsonPathReplacer
+   */
+  protected $sut;
+
+  protected function setUp() {
+    parent::setUp();
+    $this->sut = new JsonPathReplacer();
+  }
+
+  /**
+   * @covers ::replaceBatch
+   */
+  public function testReplaceBatch() {
+    $batch = $responses = [];
+    $batch[] = new Subrequest([
+      'uri' => '/ipsum/{{foo.body@$.things[*]}}',
+      'action' => 'sing',
+      'requestId' => 'oop',
+      'headers' => [],
+      '_resolved' => FALSE,
+      'body' => ['answer' => '{{foo.body@$.stuff}}'],
+      'waitFor' => 'foo',
+    ]);
+    $batch[] = new Subrequest([
+      'uri' => '/dolor/{{foo.body@$.stuff}}',
+      'action' => 'create',
+      'requestId' => 'oof',
+      'headers' => [],
+      '_resolved' => FALSE,
+      'body' => 'bar',
+      'waitFor' => 'foo',
+    ]);
+    $response = Response::create('{"things":["what","keep","talking"],"stuff":42}');
+    $response->headers->set('Content-ID', '<foo>');
+    $responses[] = $response;
+    $actual = $this->sut->replaceBatch($batch, $responses);
+    $this->assertCount(4, $actual);
+    $paths = array_map(function (Subrequest $subrequest) {
+      return $subrequest->uri;
+    }, $actual);
+    $this->assertEquals(['/ipsum/what', '/ipsum/keep', '/ipsum/talking', '/dolor/42'], $paths);
+    $this->assertEquals(['answer' => 42], $actual[0]->body);
+  }
+
+}