Add unit tests for Occurrences class

tests/Unit/OccurrencesTest.php

+<?php declare(strict_types=1);
+
+namespace ResTelae\Gbif\Tests\Unit;
+
+use PHPUnit\Framework\TestCase;
+use ResTelae\Gbif\Occurrences;
+use ResTelae\Gbif\Species;
+
+final class OccurrencesTest extends TestCase
+{
+    /** @var Occurrences */
+    private $occurrences;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->occurrences = new Occurrences();
+    }
+
+    public function testShouldGetValidResponseFromGet()
+    {
+        $response = $this->occurrences->get(1258202889);
+
+        $this->assertEquals("Alfaroa Standl.", $response['scientificName']);
+    }
+
+    public function testShouldGetValidResponseFromGetVerbatim()
+    {
+        $response = $this->occurrences->getVerbatim(1258202889);
+
+        $this->assertEquals("Alfaroa Standl.", $response['http://rs.tdwg.org/dwc/terms/scientificName']);
+    }
+
+    public function testShouldGetValidResponseFromGetFragment()
+    {
+        $response = $this->occurrences->getFragment(1258202889);
+
+        $this->assertEquals("Alfaroa Standl.", $response['scientificName']);
+    }
+}