Why do I need a database for tests in Yii2?

Question

Yii

I am familiar with testing in Yii2. I download the usual basic template, I try to run the tests, I get

Exception (Database Exception) 'yii\db\Exception' with message 'SQLSTATE[HY000] [1044] Access denied for user 'yii2'@'%' to database 'yii2_basic_tests''

The error occurs both through the console ( codecept run) and through the web ( index-test.php).
Apparently, the connection to the database in config/test.phpand is incorrectly configured config/db_test.php.
I tried to find the answer in the yii manual , but it doesn't say anything about the test database. I tried to find it in the Codeception documentation, but I couldn't either. I watched a video on YouTube, where the tests immediately ran normally without configuring the database ..
Actually, the question is: what should be specified in config/db_test.php?
1) The same database that I use in the "normal" operation of the application? What for then the separate file is necessary? Is it possible to pull data from config/db.php?
2) The second DB specially created for tests? What then it should be for a DB? Should it be empty? Does it have to be a copy of the database for the "normal" operation of the application? Here I was very embarrassed that I could not find information about this "test" database. I will be grateful if you indicate the direction where to dig.
UPD : It's also interesting why a separate config is needed at all. config/test.php
After all, when testing, the conditions should be as close as possible to the "combat" ones. Why is it impossible to use the standard config for tests?

Answer 1

A test database is needed for test manipulations, you won’t delete and add in combat, make sales and issue documents? Or will you? ;) You will not send letters to real addresses?
Test database - base-imitation. Almost like a real one, but with worthless data. Test in it.