test-new-dribble: make credentials swap asymmetric

the first case of test-new-dribble (standard-test) is updated, by making
the credentials swap between the left and right agent asymmetric.
Previously, ragent started to receive stun requests without initially
knowing lagent candidates. Now, ragent also ignores lagent credentials.
This modification allows to test changes introduced by the previous
commit.

Reviewed-by: Olivier Crête <olivier.crete@collabora.com>
Differential Revision: https://phabricator.freedesktop.org/D1890