[10.1] mariadb-test cannot be installed on sid

Technically it probably doesn't belong to MariaDB JIRA, but it affects MariaDB, so let it be here.

Debian sid provides 10.1 as mariadb-server and 10.2 as mariadb-server-10.2, same for mariadb-client / mariadb-client-10.2. However, mariadb-test points at 10.2.

As one of side-effects, our tests fail to install mariadb-test 10.1 along with 10.1 server/client.

For a note, 10.2 is 10.2.7, which is pretty old.