Add regression test for background worker restart after crash.

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 b5d084c5353,
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 <[email protected]>
Reviewed-by: ChangAo Chen <[email protected]>
Discussion: https://postgr.es/m/CAHGQGwHF-PdUOgiXCH_8K5qBm8b13h0Qt=dSoFXZybXQdbf-tw@mail.gmail.com