Improve usability of AssertMailTrait for debugging tests

rerolled to the corresponding #19 to trying to fix CCF/CI errors

Removed rerolled tag

Change looks good now

1. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -50,16 +50,56 @@ protected function getMails(array $filter = []) {
   +   * @param string $group
   +   *   An asset group.
   

   This seems unused in the method? $group existed in the past in Simpletest times, now we dropped it.

2. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -50,16 +50,56 @@ protected function getMails(array $filter = []) {
   +  protected function assertMail($name, $value = '', $message = '', $group = 'Email', $index = NULL) {
   

   Since we are here, can we typehint the parameters and/or at least the return void?

3. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -50,16 +50,56 @@ protected function getMails(array $filter = []) {
   +          ? new FormattableMarkup(
   ...
   +          : new FormattableMarkup('No email with index @index found',
   

   We now tend to avoid use of FormattableMarkup to format the assertion failure messages. Better use "No email with index {$index} found" or sprintf("No email with index %d found", $index).

4. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -50,16 +50,56 @@ protected function getMails(array $filter = []) {
   +      ? new FormattableMarkup('@message (Fails because no email found)',
   ...
   +      ? new FormattableMarkup('@message (Fails because field "@name" is not set)',
   ...
   +      : new FormattableMarkup('Field "@name" is not set', ['@name' => $name])
   

   same here

5. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -50,16 +50,56 @@ protected function getMails(array $filter = []) {
   +  protected function resetStoredEmails() {
   

   this is introduced but not used anywhere? I think we need a test, at least.

Updating patch after addressing points from #25, although i am not sure about how to move forward with point 5. please review.