Structured tests

Some tests may be done before other ones. We should provide a way to make a test dependent on an another one.

Running the "test" command will run all the tests present in the order they are found. However some tests may fail because of the same root cause leading up to a redundancy. For example if the ``interfaces`` module does not work, also the ``network`` module won't work. This is why the ``network`` test should be run only if the ``interfaces`` test does not fail.

Short description of decision:
1. The root class of StructuredTest tree is StructuredTestCase
   - This clas is responsible for processing of list of test cases that shall be executed before running particular test. Result of this processing is object of unittest.TestSuite that can be run using unittest.TestRunner in setUp method or runTest method
2. Each test case that belongs to StructuredTest group shall inherits StructuredTestCase.
    - each class should contain atrribute with name dependency_list. This is a list of modules that contains test cases that shall be run before given test case
   -each class should contain attribute test_name that contains string with name of given test case