Introduce PyTest suite migrated from `test/run` bash suite.

Migration matrix is:

`build_s2i_app` is fixture that takes `Path` test-app from `conftest.py`
file.

test_container_application.py tests all test-app directories.

test_container_basics.py tests only basics w/o application.
It also tests test_dockerfile and test_minimal_dockerfile

Signed-off-by: Petr "Stone" Hracek <phracek@redhat.com>