[FT] Run test_periodic_sync_routers_task tests serially

It's seen that these tests interfered(removed router namespace) with
test_metadata_proxy_rate_limiting_ipv6 but can interfere with others
too, let's run these serially to avoid random failures.

Following tests will run serially now:-
- test_periodic_sync_routers_task
- test_periodic_sync_routers_task_routers_deleted_while_agent_down
- test_periodic_sync_routers_task_routers_deleted_while_agent_sync

Closes-Bug: #2069744
Change-Id: I34598cb9ad39c96f5e46d98af1185992c5eb3446
(cherry picked from commit bf82263)

Author: karelyatin

tox.ini

@@ -78,8 +78,8 @@ setenv = {[testenv:dsvm-functional]setenv}
 deps = {[testenv:dsvm-functional]deps}
 commands =
   bash {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}/etc {envdir}/bin
-  stestr run --slowest --exclude-regex (.*MySQL\.|.*PostgreSQL\.|.*test_get_all_devices|.*TestMetadataAgent\.|.*BaseOVSTestCase\.) {posargs}
-  stestr run --slowest --combine --concurrency 1 (.*MySQL\.|.*PostgreSQL\.|.*test_get_all_devices|.*TestMetadataAgent\.|.*BaseOVSTestCase\.) {posargs}
+  stestr run --slowest --exclude-regex (.*MySQL\.|.*PostgreSQL\.|.*test_get_all_devices|.*TestMetadataAgent\.|.*BaseOVSTestCase\.|.*test_periodic_sync_routers_task) {posargs}
+  stestr run --slowest --combine --concurrency 1 (.*MySQL\.|.*PostgreSQL\.|.*test_get_all_devices|.*TestMetadataAgent\.|.*BaseOVSTestCase\.|.*test_periodic_sync_routers_task) {posargs}
 
 [testenv:dsvm-fullstack]
 setenv = {[testenv]setenv}