scripts/pylib/twister/twisterlib: Support multiple `--pytest-args`

One can not even replace sucessfully pytest basic sample `pytest-args`
with command line "--pytest-args", as all it does is to append a single
string to current list of commands, making it impossible to send several
arguments.

This patch fixes that by allowing several instances of `--pytest-args`
to compose the whole list of arguments to be passed to pytest.

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