tests: Fix mysql-server install on buster

test/mysql.bats

 load common
 
 begin_mysql() {
-    apt-get -qq install mysql-server
+    apt-get -qq install default-mysql-server
     zcat "${BATS_TEST_DIRNAME}/samples/bntest_p8Cz8k.sql.gz" | mysql --defaults-file=/etc/mysql/debian.cnf
     zcat "${BATS_TEST_DIRNAME}/samples/bntest_v11vJj.sql.gz" | mysql --defaults-file=/etc/mysql/debian.cnf
 }