TT#17600 pytest: import fixtures as plugins instead of importing them

Quoting from https://twitter.com/hackebrot/status/873083574676733953:

| pytest looks up fixtures based on the import path etc.. Importing
| then creates an internal copy, which messes with the cache etc.

Change-Id: I3ee6f5ab859218ce0cfe8681505b670952e47df6

t/conftest.py

@@ -1,2 +1,2 @@
 import pytest
-from fixtures import *
+pytest_plugins = ['fixtures.commandline', 'fixtures.fs', 'fixtures.gitrepo', 'fixtures.programs']