[meta] Add tests for MimeMailFormatHelper

MimeMailFormatHelper contains the core functionality of the Mime Mail module. Yet there are almost no tests, so we have no way of knowing whether the code is doing what we intend it to do, we have no way of guarding against changes, either to Mime Mail or to core Drupal, that may break this functionality.

This meta is to organize and track the effort to add tests for MimeMailFormatHelper.

There are 8 functions in this class, all of which need extensive tests. Below is the list of function with a link to the specific issue for testing that function.

1. mimeMailAddress():
   Assigned to: TR #3145176: Add test cases for MimeMailFormatHelper::mimeMailAddress() →
2. mimeMailHtmlBody():
   #3145398: Add test cases for MimeMailFormatHelper::mimeMailHtmlBody() →
3. mimeMailExtractFiles():
   #3145399: Add test cases for MimeMailFormatHelper::mimeMailExtractFiles() →
4. mimeMailFile():
   Assigned to: TR #3145400: Add test cases for MimeMailFormatHelper::mimeMailFile() →
5. mimeMailUrl():
   Test ported from D7 in Assigned to: TR #2985911: Port test cases to D8 →
6. mimeMailMultipartBody():
   #3145401: Add test cases for MimeMailFormatHelper::mimeMailMultipartBody() →
7. mimeMailRfcHeaders():
   Assigned to: TR #3145402: Add test cases for MimeMailFormatHelper::mimeMailRfcHeaders() →
8. mimeMailHeaders():
   Test ported from D7 in Assigned to: TR #2985911: Port test cases to D8 →