Fix archive restore race that could unlink WAL before rename

Hi,

KeepFileRestoredFromArchive() used to unlink an existing WAL segment
before renaming the restored file into place on non-Windows. Between
those two steps the segment is missing from pg_wal, so a concurrent
walsender (or anything else looking at that path) can observe a gap.

On POSIX, durable_rename() already replaces the target atomically, so
the prior unlink is unnecessary. Windows still needs the
rename-to-.deletedN + unlink dance, so that path is unchanged.

The attached patch drops the non-Windows unlink and adds a TAP test that
uses an injection point between the old unlink site and the rename to
show the segment remains present.

Best regards,
Stepan Neretin