Fixed exception expectation in testDeeplyNestedElements

[kuzjka]

Fixes the test for recent Java versions.

The test has two success paths:

1. Overflow limit is not hit, exception is not thrown.
2. Exception is thrown and it contains specific text, indicating that overflow protection is triggered.

Depth limit in java.xml is set by jdk.xml.maxElementDepth system property.
Default value was changed from 0 (non-limit) to 100 since jdk-19+16
So, in recent Java versions, the second path is triggered, however the check of the exception object is incorrect.

Original SAXParseException with specific text is wrapped into XMLParserException with a different message at XMLParserImpl:73

santuario-xml-security-java/src/main/java/org/apache/xml/security/parser/XMLParserImpl.java

Line 73 in d5dd9fe

throw new XMLParserException(ex, "empty", new Object[] {"Error parsing the inputstream"});

So the test needs to check the type and the message of the cause exception.

[kuzjka]

Default value was changed from 0 (non-limit) to 100 since jdk-19+16

Oops, confused with another pull request. The change in OpenJDK was introduced in jdk-24+26