Symfony 4.3 Unit Tests Not Using Test DB

I am having problems getting my Symfony 4.3 app to use a test database for unit tests.

I have been through the documentation on the Symfony website for setting up a database for testing but I must be missing something as it doesn't seem to work. The tests write data to the DB but they are still using the main DB.

I am using Sqlite and this is what I have added to .env.test

DATABASE_URL=sqlite:///%kernel.project_dir%/var/data_test.db

I have tried other methods from various sites including some answers on SO but they don't work for me.

Any help would be much appreciated.