tests: make Std{err,out}Buffer context managers stackable
tests: make Std{err,out}Buffer context managers stackable

Allow use StdoutBuffer context manager inside another similar context
manager. This allows running the tests by a test runner which collect
the output using similar trick.

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client