In basic_archive tests, insist on wal_level='replica'.

That's normally the default, but buildfarm member thorntail has
other ideas.

Nathan Bossart

Discussion: http://postgr.es/m/20220203212539.GA1082940@nathanxps13

contrib/basic_archive/basic_archive.conf

@@ -1,3 +1,4 @@
 archive_mode = 'on'
 archive_library = 'basic_archive'
 basic_archive.archive_directory = '.'
+wal_level = 'replica'