testing/nullmailer: various improvements

Squashed commit of the following:

commit af0dae99cda7c1b0c4fa725aa59a853a65644920 Author: Terra alpine-builder@spunky.tech.futurequest.net Date: Thu May 13 05:45:50 2021 -0700

commit 975ea029e145c097944f5d42c30c3d1a7882418f Author: Terra alpine-builder@spunky.tech.futurequest.net Date: Thu May 13 05:45:10 2021 -0700

testing/nullmailer: Add more comprehensive /etc/init.d/nullmailer

commit e7b93f65eeed56e6087942d93359f2d8a1199e40 Author: Terra alpine-builder@spunky.tech.futurequest.net Date: Thu May 13 05:41:56 2021 -0700

testing/nullmailer: Fix install warning regarding missing embedded checksums

  ref: https://gitlab.alpinelinux.org/alpine/abuild/-/issues/10031

commit 5bec89525c4a4a017d605ac325230cd1a9b6b3ea Author: Terra alpine-builder@spunky.tech.futurequest.net Date: Thu May 13 05:38:26 2021 -0700

testing/nullmailer: Fix various test issues

commit e288818ee33f54c798e2f279cf02612ebe3b51f0 Author: Terra alpine-builder@spunky.tech.futurequest.net Date: Thu May 13 05:34:55 2021 -0700

testing/nullmailer: Replace remotes.conf with a more descriptive version