Avoid quadratic XML report writes for ParameterizedClass

[xujiantop-crypto]

Following the Maven Surefire contribution checklist:

• The commit has a meaningful subject and body.
• The pull request description explains the behavior, cause, and fix.
• Module verification and focused tests were run locally.
• The full integration-test profile was not run locally.

Fixes #3439.

JUnit 5.14 ParameterizedClass emits one ClassSource across multiple class-template-invocation containers. RunListenerAdapter previously opened and closed the same test set for every invocation. StatelessXmlReporter then rewrote the accumulated report after each invocation, making the write cost quadratic in the number of invocations.

This keeps the report lifecycle open for a parameterized class branch and flushes the collected source reports once the class template completes. Ordinary nested class containers retain their independent lifecycles.

Validation:

• mvn -Denforcer.skip=true -DskipTests -pl surefire-providers/surefire-junit-platform -am verify
• RunListenerAdapterTest: 40 tests passed via the JUnit Platform launcher
• StatelessXmlReporterTest: 14 tests passed via the JUnit Platform launcher
• JUnit 5.14.4 sample with three @ParameterizedClass values: one XML report with tests="3" and all three invocation testcases