[3.x] Ease upgrade from 1.x to 3.x

[alexislefebvre]

Is your feature request related to a problem? Please describe.
Upgrade from 1.x to 3.x cause several issues if one doesn't read documentation carefully (like most of us do):

• Old configuration liip_functional_test.cache_sqlite_db: true causes errors, when Composer run Symfony scripts:

> Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache

In ArrayNode.php line 307:

Unrecognized option "cache_db" under "liip_functional_test" 

• Calls to loadFixtures and loadFixtureFiles throw fatal errors when running tests:

Error: Call to undefined method […]Test::loadFixtures()

Describe the solution you'd like
Errors should show how to ease upgrading

Describe alternatives you've considered

• Add a compatibility layer for old configuration, and/or throw warnings instead of a fatal error, with a link to the documentation that explain upgrade steps
• Calls to loadFixtures and loadFixtureFiles should print a text like This feature has been moved to https://github.com/liip/LiipTestFixturesBundle, please install it (but only if it doesn't create conflicts when both bundle are active)

Additional context

It would be nice if we could deprecate features before removing them, but only if its doesn't require too much work.

[Simperfit]

@alexislefebvre I need to do some more tests, but it seems that passing from 2.x to 3.x is making my test x2 longer.

[alexislefebvre]

This may be due to an issue with accesses to the container, as reported in #446.

[alexislefebvre]

Or, the use of static::$client, we can rename $client to $testClient projects' code in order to avoid the conflict: symfony/symfony#31653 (comment)