Make sure each test gets its own cache

by appending testfunction-name to cache_path

tsstats/tests/test_cache.py

@@ -13,7 +13,9 @@ from tsstats.tests.test_log import testlog_path
 
 @pytest.fixture
 def cache_path(request):
-    cache_path = 'tsstats/tests/res/tsstats.cache'
+    cache_path = 'tsstats/tests/res/tsstats.cache.{}'.format(
+        request.function.__name__
+    )
 
     def clean():
         if os.path.exists(cache_path):