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>