avocado.test: Save exception name in Test.fail_class

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.test: Save log contents in Test.text_output

At the end of a test, in order to make xunit information
richer, we can simply save the log contents of a test
in Test.text_output. It can then be recorded in the
xunit file conveniently.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>