Port mimemail_example module to D8

I don't know how much interest there is in this - I don't see any issues in the issue queue about the mimemail_example module, so it's not clear to me that anyone uses it. Likewise, I think the file attachment part of this example doesn't work in D7, so it's not clear to me that this module ever worked completely.

Regardless, here is a working port to D8 (which includes making the file attachment part work ...). This should work with D9 too.

To use, install the module then visit example/mimemail_example. This will present you with a form you can fill in to test e-mail sending with Mime Mail.

*I* will find this module useful, as it can be used to test a site to verify that mail is set up properly, and it can be used during the development of Mime Mail to test operation of the module and bug fixes/new features. I guess it also serves as an example of how to programmatically send e-mail with Mime Mail.

Tests ran green, but because we have no explicit tests for mimemail_example that doesn't prove much other than there are no coding standards problems or syntax error or other gross things like that.

Added a "Needs tests" tags. It would be nice, and extremely helpful, to have at least one Functional test which enables this module and uses the example/mimemail_test to send an e-mail to a test email collector. That way we would have a way to verify via DrupalCI that sending Mime Mail works properly.

I'm just going to add one simple test for this module, as a framework for more extensive tests. Additional testing can be handled in a separate issue.

I think this is a testbot issue - it doesn't seem that the mimemail_example module is getting installed before the test is run, because the testbot builds the test environment using the .info.yml files in the repository (NOT the ones in the patch), and mimemail_example.info.yml doesn't exist in the repository yet. That's my working theory - this test is so simple, and it runs fine locally that I can't imagine there's anything wrong with the test. I'm just going to commit it and see - I can always fix things later if this is not the case.

OK, that theory was wrong I guess. I reverted the commit that contained the test, so this won't break the branch tests. I will have to do some experimenting with the testbot to see why this test works locally but fails on the testbot.

Splitting off the tests into a separate issue #3128807: Add tests for the mimemail_example module

The mimemail_example module has been ported and committed. Please try it out and if you find any problems open a new issue.