[qa/rpc-tests] A test base class instead of skeleton.py

Motivation - skeleton.py is a template for RPC tests. instead of creating a copy each time someone creates a new test (difficult to maintain and update fixes), new tests can simply inherit the Skeleton class in skeleton.py.

What i did - 
1) Changed skeleton.py so all methods are inside a function
2) Changed listtransactions.py to inherit from the Skeleton class, now only implements run_test
3) added an exampe test file (TEST_EXAMPLE.py) to serve as a template for future tests.