Remove unused test dependencies org.reflections & maven-rewrite

[Philzen]

This PR:

• removes dependency org.reflections
  …is not used anywhere
• remove unused code and some minor optimizations for TestUtils
• covers TestUtils.validatePomXmlFiles(List) with tests
  Yo, dawg, i heard you like tests, so i wrote some tests for your test helpers 😉 (well really it was to make sure the following step produced runtime behavior)
• Reimplemented the method with Jackson XML mapper
  Although the RawPom.parse method implements tight checks by databinding to its own sophisticated pom POJO, it is really only the five assertions in TestUtils.validatePomXmlFiles(List) that we're after. rewrite-maven imports a whopping 73 MB onto the classpath with dozens of transitive dependencies having to be gulped by our IDEs – a little overkill just for a single test helper method.
• Remove org.openrewrite:rewrite-maven and keep jackson-dataformat-xml explicitly (was a transitive dependency of rewrite-maven)
  NB: Open Rewrite (big fan here BTW) does not need to be included in the pom in order to use it – one can just use the "Maven command line" option tab on the recipe pages.

All in all, this brings down the number of total dependencies in the openapi-generator module from 143 to 105 🚀

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[wing328]

lgtm. thanks for the enhancement.

fyi @OpenAPITools/generator-core-team