Requirement test samples should be accepted as *.xml files rather than *.log

[hsekowski-splunk]

The files content has to be XML formatted
Beside making sure this is true once tests are run (there is separate issue raised for that #548), the test should accept *.xml files, so that anyone can easily validate them - eg. by opening in web browser.

This enhancement can be implemented in two or three stages:

1. accept both formats: *.log and *.xml, that would require non-breaking change
2. WARN is *.log is used with information that expected is *.xml and *.log will be depreciated
3. accept only *.xml