docs: add @codeCoverageIgnore

Can't be covered on codecov.io
https://codecov.io/gh/kenjis/phpunit-helper/src/fbc1cc8756642f483e1cff06d69a53d4745799ed/src/functions.php

Author: kenjis

src/functions.php

@@ -10,6 +10,7 @@
  * @SuppressWarnings(PHPMD.ExitExpression)
  * Does not work if I move to the function annotation
  * See https://github.com/phpmd/phpmd/issues/337
+ * @codeCoverageIgnore Can't cover on codecov.io
  */
 if (! function_exists('dd')) {
     /**
@@ -27,6 +28,9 @@ function dd(...$vars): void
     }
 }
 
+/**
+ * @codeCoverageIgnore Can't cover on codecov.io
+ */
 if (! function_exists('d')) {
     /**
      * Alias of var_dump()