Fix coverage unnecessary files and functions to cover

Author: terremoth

phpunit.xml

@@ -19,5 +19,8 @@
         <include>
             <directory>src</directory>
         </include>
+        <exclude>
+            <file>src/Terremoth/Async/background_processor.php</file>
+        </exclude>
     </source>
 </phpunit>

src/Terremoth/Async/script_functions.php

@@ -2,6 +2,10 @@
 
 namespace Terremoth\Async;
 
+/**
+ * @codeCoverageIgnore
+ * @codeCoverageIgnoreStart
+ */
 function customError(string $error): void
 {
     $error = 'Error: ' . $error;