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_pop3.bats

@@ -33,11 +33,10 @@ function teardown_file() { _default_teardown ; }
   assert_success
 }
 
-@test '/var/log/mail/mail.log is error-free' {
-  _run_in_container grep 'non-null host address bits in' /var/log/mail/mail.log
-  assert_failure
-  _run_in_container grep ': error:' /var/log/mail/mail.log
-  assert_failure
+# TODO: Remove in favor of a common helper method, as described in vmail-id.bats equivalent test-case
+@test 'Mail log is error free' {
+  _service_log_should_not_contain_string 'mail' 'non-null host address bits in'
+  _service_log_should_not_contain_string 'mail' ': Error:'
 }
 
 @test '(Manage Sieve) disabled per default' {