Fix PHPStan errors from UserCreationTrait and AssertMailTrait

Widening the scope slightly to incorporate the other trait that causes issues like this. Crediting @mondrake since this is similar to the work in #3319582: Fix calls to methods with too many parameters passed in

And have patches for both 10.0.x and 10.1.x

1. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -82,13 +82,8 @@ protected function assertMail($name, $value = '', $message = '', $group = 'Email
   -   * @param string $group
   -   *   (optional) The group this message is in, which is displayed in a column
   -   *   in test output. Use 'Debug' to indicate this is debugging output. Do not
   -   *   translate this string. Defaults to 'Other'; most tests do not override
   -   *   this default.
       */
   -  protected function assertMailString($field_name, $string, $email_depth, $message = '', $group = 'Other') {
   +  protected function assertMailString($field_name, $string, $email_depth, $message = '') {
   

   Now that we have PHPStan level 1 any calls to assertMailString that have 4 arguments will produce an error - so we don't need to trigger a deprecation here.

2. +++ b/core/lib/Drupal/Core/Test/AssertMailTrait.php
   @@ -122,20 +117,15 @@ protected function assertMailString($field_name, $string, $email_depth, $message
   -   * @param string $group
   -   *   (optional) The group this message is in, which is displayed in a column
   -   *   in test output. Use 'Debug' to indicate this is debugging output. Do not
   -   *   translate this string. Defaults to 'Other'; most tests do not override
   -   *   this default.
       */
   -  protected function assertMailPattern($field_name, $regex, $message = '', $group = 'Other') {
   +  protected function assertMailPattern($field_name, $regex, $message = '') {
   

   As above.

This patch shrinks core/phpstan-baseline.neon from 199K to 150K... not bad.