Rewrite Text.xml parsing to reduce memory usage

The current test report logic requires 2-3 times the CTest Test.xml report size in memory.

This means that with some specific projects we may need up to 2 GB of RAM, and the server gets killed by OpenShift.

Despite some mitigations already in place, the only proper way to reduce the memory use is to replace the current DOM based XML parsing (ElementTree) with some XML stream parser.