Add regression test for background worker restart after crash.

Previously, if a background worker crashed and the server restarted
with restart_after_crash enabled, the worker was not restarted
as expected. This issue was fixed by commit b5d084c535,
which ensures that background workers without the never-restart flag
are correctly restarted after a crash-and-restart cycle.

To guard against regressions, this commit adds a test that verifies
a background worker successfully restarts in such a scenario.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: ChangAo Chen <cca5507@qq.com>
Discussion: https://postgr.es/m/CAHGQGwHF-PdUOgiXCH_8K5qBm8b13h0Qt=dSoFXZybXQdbf-tw@mail.gmail.com

src/test/recovery/t/013_crash_restart.pl

@@ -228,6 +228,13 @@ is( $node->safe_psql(
 	'before-orderly-restart',
 	'can still write after crash restart');
 
+# Confirm that the logical replication launcher, a background worker
+# without the never-restart flag, has also restarted successfully.
+is($node->poll_query_until('postgres',
+	"SELECT count(*) = 1 FROM pg_stat_activity WHERE backend_type = 'logical replication launcher'"),
+	'1',
+	'logical replication launcher restarted after crash');
+
 # Just to be sure, check that an orderly restart now still works
 $node->restart();