Add tests to the crate

[Nightloewe]

As all libraries need to be tested, we should implement test cases for everything

• Session::is_expired. ::update, ::invalidate, ::insert, ::remove, ::get_attribute
• Test LocalSessionMap::*
• Test SessionStore

if rocket feature is enabled:

• Test CookieSessionConfig::id_gen
• Test CookieSession::access, ::from_request
• Test Sessions and SessionFairing