lava: fix resubmit() function
lava: fix resubmit() function

When resubmitting the job, old test run was assigned to new test job.
This created confusion in the web UI. This patch makes the testrun field
in TestJob object empty when resubmitting. Fixes #228.

Signed-off-by: Milosz Wasilewski <a39812e55748be210556797addcf4c064734f1bf@linaro.org>

terceiro/squad,terceiro/squad,terceiro/squad,terceiro/squad