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>

doc/develop/test/pytest.rst

@@ -166,6 +166,22 @@ a fixture ``mcumgr``
       mcumgr.reset_device()
       # continue test scenario, check version etc.
 
+
+unlauched_dut
+=============
+
+Similar to the ``dut`` fixture, but it does not initialize the device. It can be used when a finer
+control over the build process is needed. It becomes responsibility of the test to initialize the
+device.
+
+.. code-block:: python
+
+   from twister_harness import DeviceAdapter
+
+   def test_sample(unlauched_dut: DeviceAdapter):
+      unlaunched_dut.launch()
+      unlaunched_dut.readlines_until('Hello world')
+
 Classes
 *******