[FrameworkBundle] Added unit-tests for GlobalVariables::getUser()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Tests added should explain that `getUser()` should return `null` when a string is found as user. If this is not correct, a PR should be made. However, this would result in a huge BC break due to people using `{% if app.user %}` which would return `null` if an anonymous token was found. If this suddenly returns a string, this check will fail.

While at it, I have also added `getUser()` tests to verify the unhappy flow is working. These tests uncovered that  if `$container->get('security.token_storage')` fails, it will throw an exception rather than return `null`. This issue is now fixed.

List of changes
--------------------
- The old `testGetUser` has been refactored to be tested with multiple variations of return types to verify the return type to work as the code tells. 
- `get('security.token_storage')` is now only executed if `has('security.token_storage')` returns true

@fabpot I think this PR should be merged before 2.6, because it fixes an uncaught exception bug in my previous PR which splits the security context
this does not make sense IMO. ``$token->getUser()`` cannot return a TokenInterface