self.avocado.plugins.runner: don't need to quote list of tests.

The parser for the subcommand run now accepts one or more
tests ids (paths / aliases) and there isn't a need to quote the list
as a string.

Example:

* Before: avocado 'sleeptest failtest /usr/bin/true'
* Now: avocado sleeptest failtest /usr/bin/true

So it's now possible to run avocado like `avocado run *.py`.

Signed-off-by: Ruda Moura <rmoura@redhat.com>