Add condition in .simpletest-db for mariadb

Migrated issue

Reported by: dimitriskr

Related to !129 (merged)

Problem/Motivation

if _TARGET_DB_TYPE is mariadb, the SIMPLETEST_DB variable is not populated, making tests fail.

Steps to reproduce

Add _TARGET_DB_TYPE: "mariadb" as a variable to a gitlab-ci.yml

Proposed resolution

Add a condition for when _TARGET_DB_TYPE is "mariadb"

Remaining tasks

User interface changes

API changes

Data model changes