Fix GH-13517: Multiple test failures when building with --with-expat

The reflection failure is because the XML extension is used to check the
module dependency information, but that exte

Fix GH-13517: Multiple test failures when building with --with-expat

The reflection failure is because the XML extension is used to check the
module dependency information, but that extension can be configured to
not depend on ext/libxml, resulting in a different output. The solution
is to check another extension instead.

The test failures in ext/xml/tests are because of different behaviour
between libxml2 and Expat error handling. These are expected differences
and the solution is to split the tests.

Closes GH-13522.

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