Set wait-timeout to proper default (28800)

Older (than 10.5) MariaDB docker containers come
with wait-timeout = 600 and that makes unit tests
execution to fail with "MySQL is gone away".

Apparently this is already ok with current 10.5
so current moodle-docker instances are passing ok
without needing the setting

But, no matter of that, we are explicitly adding
the setting here, to enable anybody wanting to use
older MariaDB docker images to get tests passing.

See https://tracker.moodle.org/browse/MDLSITE-6141
for more details.

Author: stronk7

db.mariadb.yml

@@ -10,6 +10,7 @@ services:
               --character-set-server=utf8mb4
               --collation-server=utf8mb4_bin
               --innodb_file_per_table=On
+              --wait-timeout=28800
     environment:
       MYSQL_ROOT_PASSWORD: "m@0dl3ing"
       MYSQL_USER: moodle