xml: update entity parse test

[ttytm]

Upgrades an xml test that assigns values to the private field parsed_reverse_entities which in an external test. Otherwise, the fields data is only used internally.

v/vlib/encoding/xml/types.v

Lines 61 to 63 in 254250d

	struct Prolog {
	parsed_reverse_entities map[string]string = default_entities_reverse.clone()
	pub:

The new test might be more robust and easier to follow as well, as it compares the parses files with entities with an an expected file that have the entities already resolved.

The upgrade is also required for #21183

[spytheman]

Good work.