cubicweb #3749378 CWTC.init_config not called before startup hooks are called [validation pending]

this is problematic when a cube use a required option in a startup hook, there is no way to set it before it gets called, resulting in a KeyError in the hook.

prioritynormal
typebug
done in3.19.2
load0.200
load left0.000
closed by#0872ac2a1db0 [testlib] call init_config once the config has been properly bootstraped
patch[testlib] call init_config once the config has been properly bootstraped [applied]