twister: pytest: Move fixtures to one file

Fixtures in pytest-twister-harness plugin are moved to one
file to simplify adding new fixtures in the future - no
need to add pytest_plugins entry and register asserts.
Moved also shell fixture from sample dir, because that
fixture can be reused in new tests.

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