sanitycheck: Add a feature which can handle pytest script.

1. Add a class of Pytest(Harness) which can handle pytest script
in harness.py
2. Use running_dir to store current test directory which be
used in pytest scrpt.
3. Add usage of this feature into zephyr doc.

Signed-off-by: YouhuaX Zhu <youhuax.zhu@intel.com>