PyExecutionFixtureTest contract is cleaned and documented.

PyExecutionFixtureTest contract is cleaned and documented.

PyExecutionFixtureTest been used incorrectly in many tests. It is now documented (see class doc) how to use it, and tests fixed to work according to this contract.

Generally, each inheritor provides path to its test data (folder on local fs, checked from vcs) and PyExecutionFixtureTest then copies it to fixuture's temp folder. After it, test is encouraged to use only temp folder, and never access real test data directly.