- Renaming to prevent conflicts with the existing method in
the newer PHPUnit used in PHP 8.
- Making it static because "assertIsInt" is static too, so it'll be
easier to replace in the future.
Previously, if the product didn't have an explicit low stock value
amount the value of the woocommerce_notify_low_stock_amount option,
which is a string, was returned verbatim.
Also, update related unit tests to create the option value as a string,
and to check that the value returned by woocommerce_notify_low_stock_amount
is always an integer.