Introduce new TestEnvironment usable by all binary crates

[syphar]

• migrate the test environment structs for all binary crates into a single one
• library crates keep (for now) their smaller custom test envs
• introduce AppConfig trait (needed for the shared test env), migrate all config structs to use it

smaller design changes:

• TestEnvironment can Deref into Context, so some duplicated methods go away
• some attributes on the TestEnvironment are now private, people need to use the deref or the accessor methods.

not in here, but I want to do it separately:

• more config builders, currently test config building is all over the place
• more docstrings / design explanation

[syphar]

@GuillaumeGomez next refactor / cleanup :)