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>

scripts/schemas/twister/testcase-schema.yaml

@@ -62,6 +62,9 @@ mapping:
           "repeat":
             type: int
             required: no
+          "pytest_root":
+            type: str
+            required: no
           "regex":
             type: seq
             required: no
@@ -187,6 +190,9 @@ mapping:
               "repeat":
                 type: int
                 required: no
+              "pytest_root":
+                type: str
+                required: no
               "regex":
                 type: seq
                 required: no