Option memory of SQLite not working

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Hello, in Symfony 3.4, I tried using the `memory` option in order to make my tests faster but an error occurred:

```
1) Tests\AppBundle\Controller\DefaultControllerTest::testIndex
copy(mydb): failed to open stream: No such file or directory
```

My test config is the following:

```
imports:
- { resource: config_dev.yml }

framework:
test: ~
session:
storage_id: session.storage.mock_file
name: MOCKSESSION
profiler:
collect: false

web_profiler:
toolbar: false
intercept_redirects: false

swiftmailer:
disable_delivery: true

liip_functional_test:
cache_db:
sqlite: liip_functional_test.services_database_backup.sqlite

doctrine:
dbal:
default_connection: default
connections:
default:
driver: pdo_sqlite
memory: true
security:
firewalls:
main:
http_basic: ~
```

Thank you.

Start by reproducing the reported Symfony 3.4 configuration, focusing on the liip_functional_test cache_db SQLite service and the Doctrine DBAL memory option. Trace the source of the `copy(mydb)` failure and verify that functional tests using an in-memory SQLite database run without attempting to copy a file.

Written by the indexing model from the issue text.