avocado.plugins.xunit: Fix incorrect double quoting

The extra double quotes left by mistake in the error
reporting code made the XML invalid. Fix that, and make
sure it stays that way with follow up functional tests.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
selftests/functional: Add xunit plugin functional tests

Run avocado with the xunit output plugin, parsing and
verifying the sanity of the outputs.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.crypto: Fix return value of get_random_string
tests: Add trinity wrapper

Trinity is a popular linux system call wrapper

http://codemonkey.org.uk/projects/trinity/

By implementing it we noticed a number of shortcomings
with avocado that have to be fixed in order to provide
a smoother developer experience.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.test: If main test action fails, cleanup still needs to be executed

This was quite the brown paper bag bug I made: If the
main action fails, we need to call cleanup() no matter
what.

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