Part of #9744

Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now.

`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl&gt;`>

Signed-off-by: Manuel Stahl <manuel.stahl@awesome-technologies.de>

This is in preparation for having multiple data stores that offer
different functionality, e.g. splitting out state or event storage.

Fix the test to pass the right number of args to the Store constructors

setup_test_homeserver function in utils.

Neaten more of the storage layer tests with assertObjectHasAttributes; more standardisation on test layout

Need to prepare() the SQLiteMemoryDbPool before passing it to HomeServer constructor, as DataStore's constructor will want it ready