Adding auto-generated tests cases from clients of Libxml2

Hi,

In this PR I have added 3 test cases to runtest.c. Those test cases were automatically generated from usages of libxml2 APIs as seen in xmlsec (https://www.aleksey.com/xmlsec/). The idea here is to add test cases to libxml2 that represent how clients of the library use the APIs. In this PR we identified 3 test cases that are worthy of contribution to libxml2. All the test cases use the same values as used by xmlsec. The values used in the test cases come from actual values used in xmlsec, particularly they were extracted by running the regression tests of xmlsec.

XMLSEC is under MIT license like libxml2 as far as I understand which means there should not be any problem regarding licensing here.

I would be really interested in hearing your feedback on those test cases.