test-network: various cleanups

test-network: various cleanups

- introduce several helper functions
- do not list unit files, but remove the runtime unit directory in
  tearDown().
- do not list used interfaces, but remove all interfaces previously not
  exists in tearDown().
- save routes and routing policy rules before running tests, and flush
  unnecessary routes and rules in each tearDown() calls.
- drop many time.sleep() calls.
- call tearDown() after each sub tests.
- shorten code.
- several coding style fixes.
- etc, etc...

Hopefully, this improves performance of the test.