test: speedup mysql and postgres integration tests by 10-20x

Use docker tmpfs mounts for mysql and postgres data volumes

docker/blocks/mysql_tests/docker-compose.yaml

@@ -7,3 +7,4 @@
       MYSQL_PASSWORD: password
     ports:
       - "3306:3306"
+    tmpfs: /var/lib/mysql:rw

docker/blocks/postgres_tests/docker-compose.yaml

@@ -5,3 +5,4 @@
       POSTGRES_PASSWORD: grafanatest
     ports:
       - "5432:5432"
+    tmpfs: /var/lib/postgresql/data:rw
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