[testlib] make a clear distinction between input / output HTTP headers The get_header() / set_header() API is the same as for CubicWebRequestBase : - get_header() returns the **request** header - set_header() / add_header() works on the **response** header FakeRequest adds two additional methods : - get_response_header() returns the **reponse** header - set_request_header() adds a **request** header FakeRequest now uses the `headers_out` ineherited from CubicWebRequestBase. It also uses a `_headers_in` attribute

[entity] create_entity/cw_instantiate should initialize entity's cache to avoid later queries

[testlib] refactor create_user and grant_permission to make them usable from pre_setup_database

[3.12 regression] don't access to __class__ directly, it may raise AttributeError

[testlib] fix typos / NameErrors in devtools/__init__.py

[schema serialization] add warning on missing group

backport stable

[testlib] fix bug in backup file name generation: it may erroneously replace '-' by '_' in database **parent directories**. This should fix random error when test are run under apycot.