Twister: Rework integration with renode-test

Initial integration with renode-test was introduced in commit bdf02ff5,
which added support for calling the `renode-test` command from both west
and twister.

This commit removes the custom run_renode_test target used for running
Robot tests with the `west build` command and makes twister call
`renode-test` directly instead.

Signed-off-by: Michał Szprejda <mszprejda@antmicro.com>