dovetail tool: replace the hard-coded "Tescase" and modify some classmethod

dovetail tool: replace the hard-coded "Tescase" and modify some classmethod

JIRA: DOVETAIL-45

1. In the testcase.py file, we replace the hard-coded "Tesecase" with "cls" or "self" based on
   the specific contexts.
2. For all the four committed files, we modify some classmethod into staticmethod because each
   method is basically just a function, called syntactically like a method, but without access
   to the object and its' internals (attributes and other methods).

Change-Id: Ieb452f476a2d33ae9aca4c904ae7d2c92b68689e
Signed-off-by: Linghui Zeng <linghui.zeng@huawei.com>