Deterministic connection collation in tests

The default collation for utf8mb4 is changed to utf8mb4_0900_ai_ci in
MySQL 8.0, so `test_mysql_connection_collation_is_configured` is
affected by MySQL version and/or default_collation_for_utf8mb4 system
variable.

https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_default_collation_for_utf8mb4

```
% ARCONN=mysql2 be ruby -w -Itest test/cases/adapters/mysql2/connection_test.rb -n test_mysql_connection_collation_is_configured
Using mysql2
Run options: -n test_mysql_connection_collation_is_configured --seed 10421

# Running:

F

Failure:
Mysql2ConnectionTest#test_mysql_connection_collation_is_configured [test/cases/adapters/mysql2/connection_test.rb:108]:
Expected: "utf8mb4_general_ci"
  Actual: "utf8mb4_0900_ai_ci"

rails test test/cases/adapters/mysql2/connection_test.rb:106

Finished in 0.013842s, 72.2450 runs/s, 144.4900 assertions/s.
1 runs, 2 assertions, 1 failures, 0 errors, 0 skips
```

activerecord/test/config.example.yml

@@ -59,6 +59,7 @@ connections:
     arunit2:
       username: rails
       encoding: utf8mb4
+      collation: utf8mb4_general_ci
 
   oracle:
      arunit: