tests: new sending and filtering functions (#3786)

* move log/filter functions into own file

* add ShellCheck global directives

* use new function for tracking logs

The new function, called `_send_email_with_mid`, aligns with suggestions
from @polarethene and is heavily simplified compared to its predecessor
`_send_email_and_get_id`. New helpers will be introduced to filter logs
according to the MID constructed in this function.

* new filters for searching logs with MID

* use new filters (and sending) functions

* add new helper for asserting non-existence of log message

* use new filters in tests

* Apply suggestions from code review

- `_mid` / `MID` => `_msgid` / `MSG_ID`
- Revised documentation / tooltip comments

* Apply suggestions from code review

* fix tests

* use more distinct names for MSG_ID headers

* update `_filter_service_log` to not use `-i -E`

Moreover, I added a function to print the whole mail log. Appropriate
comments were added to this function to indicate that one should only
use this function when necessary.

* adjust helpers to new helper filter

* follow-up of previous commit

* add CHANGELOG entry

* Apply suggestions from code review

* chore: Update OAuth2 to use new log helper

* Apply suggestions from code review

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

* added explicit `_regexp` filters for logs

* Apply suggestions from code review

---------

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

test/tests/serial/mail_with_oauth2.bats

@@ -110,7 +110,7 @@ function __verify_auth_with_smtp() {
 
 function __dovecot_logs_should_verify_success() {
   # Inspect the relevant Dovecot logs to catch failure / success:
-  _run_in_container grep 'dovecot:' /var/log/mail.log
+  _service_log_should_contain_string 'mail' 'dovecot:'
   refute_output --partial 'oauth2 failed: Introspection failed'
   assert_output --partial "dovecot: imap-login: Login: user=<${USER_ACCOUNT}>, method=${AUTH_METHOD}"