avocado.test: Save test traceback in Test.traceback

If a traceback was generated while executing an
avocado test, save the text contents to the
traceback attribute. It can then be used by code
such as the xunit plugin, to store that info.

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>