refactor: move docs and compose config for database tests

Those files are only realted to `tests\database` tests.

tests/databases/README.md

@@ -0,0 +1,47 @@
+# Persistence Tests
+
+Torrust requires Docker to run different database systems for testing.
+
+Start the databases with `docker-compose` before running tests:
+
+```s
+docker-compose -f tests/databases/docker-compose.yml up
+```
+
+Run all tests using:
+
+```s
+cargo test
+```
+
+Connect to the DB using MySQL client:
+
+```s
+mysql -h127.0.0.1 -uroot -ppassword torrust-index_test
+```
+
+Right now only tests for MySQLite are executed. To run tests for MySQL too,
+you have to replace this line in `tests/databases/mysql.rs`:
+
+```rust
+
+```rust
+#[tokio::test]
+#[should_panic]
+async fn run_mysql_tests() {
+    panic!("Todo Test Times Out!");
+    #[allow(unreachable_code)]
+    {
+        run_tests(DATABASE_URL).await;
+    }
+}
+```
+
+with this:
+
+```rust
+#[tokio::test]
+async fn run_mysql_tests() {
+    run_tests(DATABASE_URL).await;
+}
+```