add a context file for tests

With the context file on can import modules from the main package. This is very
helpful for tests.

Just do, import context in each test to activate it.

tests/context.py

@@ -0,0 +1,4 @@
+import os
+import sys
+sys.path.insert(0, os.path.abspath(os.path.join(os.path.dirname(__file__),
+                                                '../borg_qt')))