ci: enable LAVA boot and smoke tests on qemu devices for new MRs

Given test scope for new MRs is not defined yet and we only have one
rpi4, b68 and c61 in the warsaw lab which are occupied mostly by daily
scheduled pipeline, starting with boot and smoke tests on qemu devices
should be a good starting point for per MR testing.

Jobs that extend `.lava-test` will be added to MR's pipeline and
schedueld pipeline.

`.lava-test-scheduled` extends `.lava-test` with new scheduled pipeline
only rule. Long run jobs like ltp should extends the job so that they
wouldn't prevent MRs from merging.

Solves #486.

Signed-off-by: Chase Qi <chase.qi@linaro.org>