Add tests

tests/CronBuilderTest.php

@@ -45,6 +45,30 @@ public function it_builds_with_prod_env_set(): void
         self::assertSame($expected, $crontab);
     }
 
+    /**
+     * @test
+     */
+    public function it_merges(): void
+    {
+        $existingCrontab = <<<CRON
+0 0 * * * /usr/bin/php /home/johndoe/public_html/other-job.php
+
+###> Automatically generated by Setono Cron Builder - DO NOT EDIT ###
+0 0 * * * /usr/bin/php /home/johndoe/public_html/delete-files.php
+###< Automatically generated by Setono Cron Builder - DO NOT EDIT ###
+CRON;
+
+        $expected = <<<CRON
+0 0 * * * /usr/bin/php /home/johndoe/public_html/other-job.php
+
+###> Automatically generated by Setono Cron Builder - DO NOT EDIT ###
+0 0 * * * /usr/bin/php /home/johndoe/public_html/send-report.php # Run every day at midnight
+###< Automatically generated by Setono Cron Builder - DO NOT EDIT ###
+CRON;
+
+        self::assertSame($expected, CronBuilder::merge($existingCrontab, self::getCronBuilder()));
+    }
+
     private static function getCronBuilder(): CronBuilder
     {
         return (new CronBuilder())