tests(chore): `tests.bats` - Remove redundant config
===
- ONEDIR volume support no longer relevant, this should have been dropped.
- ClamAV ENV no longer relevant as related tests have been extracted already.
- Same with the some of the SpamAssassin ENV config.
- `VIRUSMAILS_DELETE_DELAY` is tested in the file, but doesn't use this ENV at all? (runs a separate instance to test the ENV instead)
- Hostname updated in preparation for migrating to new test helpers. Relevant test lines referencing the hostname have likewise been updated.

tests(chore): `tests.bats` - Convert to common setup
===
ENV remains the same, but required adding `ENABLE_AMAVIS=1` to bring that back, while the following became redundant as they're now defaulting to explicitly disabled in the helper method:

- `ENABLE_CLAMAV=0`
- `LOG_LEVEL=debug`
- `ENABLE_UPDATE_CHECK=0`
- `--hostname` + `--tty` + standard `--volume` lines
- `-e` option expanded to long-name `--env`, and all `\` dropped as no longer necessary.

`wait_for_finished_setup_in_container` is now redundant thanks to `common_container_setup`.

tests(refactor): `tests.bats` - Extract out Dovecot Sieve tests
===
Sieve test files relocated into `test/config/dovecot-sieve/` for better isolation.

`dovecot.sieve` was not using the `reject` import, and we should not encourage it? (docs still do):
https://support.tigertech.net/sieve#the-sieve-reject-jmp

tests: `tests.bats` - Extract out `checking smtp` tests
===
Migrated to the standard template and copied over the original test cases with `_run_in_container` adjustment only.

Identified minimum required ENV along with which mail is required for each test case.

tests(refactor): `smtp-delivery.bats`
===
- Disabled `ENABLE_SRS=1`, not necessary for these tests.
- Added a SpamAssassin related test (X-SPAM headers) which requires `SA_TAG` to properly pass (or `ENABLE_SRS=1` to deliver into inbox).
- Many lines with double quotes changed to single quote wrapping, and moving out `grep` filters into `assert_output --partial` lines instead.
- Instead of `wc -l` making failures less helpful, switch to the helper method `_should_output_number_of_lines`
- x2 `assert_output` with different EOF style of usage was not actually failing on tests when it should. Changed to assert partial output of each expected line, and count the number of lines instead.
- Added additional comments related to the test cases with a `TODO` note about `SPAMASSASSIN_SPAM_TO_INBOX=1`.
- Revised test case names, including using the common prefix var.
- `tests.bats` no longer needs to send all these emails, no other test cases require them. This affects a test checking a `/mail` folder exists which has been corrected, and a quotas test case adjusted to expect an empty quota size output.

tests: `tests.bats` - Extract out test cases for config overrides
===
Slight improvement by additionally matching `postconf` output to verify the setting is properly applied.

tests: `tests.bats` - Extract out Amavis SpamAssassin test case
===
Removes the need for SpamAssassin ENV in `tests.bats`.