core: Use ap_size_parameter_parse() for LimitXMLRequestBody

[arturobernalg]

LimitXMLRequestBody got its own apr_strtoff() parsing in #825, before ap_size_parameter_parse() was added in #829. Use the common helper like LimitRequestBody does. No change in the accepted values.

Add LimitXMLRequestBody to the directives checked in test_012.

---

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  (see SECURITY.md and
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[arturobernalg]

Superseded by #848, which includes this change together with the fix of ap_size_parameter_parse() itself.