twister: pytest: Allow using pytest-args from command file and yaml

Extend pytest-args from configuration yaml file with args
from command line instead of overwriting.

Signed-off-by: Grzegorz Chwierut <grzegorz.chwierut@nordicsemi.no>

doc/develop/test/pytest.rst

@@ -97,6 +97,8 @@ There are two ways for passing extra arguments to the called pytest subprocess:
       -s samples/subsys/testsuite/pytest/shell/sample.pytest.shell \
       --pytest-args='-k test_shell_print_version'
 
+   The command line arguments will extend those from the .yaml file. If the same argument is
+   present in both places, the one from the command line will take precedence.
 
 Fixtures
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