Began support of XML-formatted logs for unit tests.

* jamroot:
  * Added `<cradle-xunit-log-format>` and `<cradle-xunit-log-format-run>`
    features.
  * Added `CRADLE_XUNIT_LOG_FORMAT_RUN_PROPERTIES` hook.
  * Added `CRADLE_XUNIT_LOG_FORMAT_RUN_PROPERTIES` to
    `CRADLE_RUN_PROPERTIES` hook.
* test/unit/exception/jamfile: Tweaked comments.
* test/unit/exception/uncaught_exception_run.cpp: Added a global fixture to
  redirect the XML-formatted log output.