twister/pytest: Add unlaunched_dut fixture

Add a new fixture, `unlaunched_dut`, which is basically the `dut`
one, but without launching the device - thus also not building the
application. It is useful for tests who need a finer control of
the building of the application, before the dut can be launched
to run it.

It will be used on a future patch, which will use it to enable LLEXT EDK
tests.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>