Run mysql tests

Not using containers (yet), for now just setting up MariaDB with an
unprivileged user and seeing how that goes. Postgres and Firebird also
seem doable, although SQL Server for ODBC definitely won't be.

.github/actions/test-gentoo/action.yml

@@ -9,7 +9,15 @@ runs:
     - shell: bash
       run: |
         set -x
-        # XXX: Set up database tests?
+        # XXX: Set up other database tests?
+        # XXX: These tests are not running containerized
+        export MYSQL_TEST_USER=ci
+        export MYSQL_TEST_PASSWD=ci
+        if [[ -z "$PDO_MYSQL_TEST_DSN" ]]; then
+          export PDO_MYSQL_TEST_DSN="mysql:host=localhost;dbname=test"
+        fi
+        export PDO_MYSQL_TEST_USER=ci
+        export PDO_MYSQL_TEST_PASS=ci
         # Slow tests criteron is doubled because this runner isn't as fast as others
         export SKIP_IO_CAPTURE_TESTS=1
         export STACK_LIMIT_DEFAULTS_CHECK=1