* Auth: Do not search for the user twice

Previously `initContextWithBasicAuth` did not use `LoginUserQuery`, doing
`GetUserByLoginQuery` only i.e. looking user in DB only, things changed when
this function started to check LDAP provider via `LoginUserQuery` (#6940),
however, this request was placed after `GetUserByLoginQuery`, so we first
looking in DB then in the LDAP - if LDAP user hasn't logged in we will
not find it in DB, so `LoginUserQuery` will never be reached.

`LoginUserQuery` request already performs `GetUserByLoginQuery`
request in correct sequence. So we can just remove redundant request.

* Correct sequence execution during authentification &
  introduce tests for it

* Move basic auth tests to separate test file, since main test file already
  pretty large

* Introduce `testing.go` for the middleware module

* Remove redundant test helper function

* Make handler names more explicit

Ref 5777f65d
Fixes #18329

* Auth: address review comment