Allow passing a block to the create_inbound_email_from_mail and
receive_inbound_email_from_mail test helper methods.
When you wanted to test a multipart email -- for example, an email that
has both a plaintext part and an HTML part -- there wasn't a way to
easily build one without a pre-made fixture. By allowing you to pass a
block to these methods, we unlock the power of Mail.new to easily
create a variety of emails, from multi-part emails, to emails with
attachments, and beyond.
`receive_inbound_email_from_source` should accept an argument list
(`*args`) instead, to allow for the `source` argument in
`create_inbound_email_from_source`.
```ruby
receive_inbound_email_from_source(source, status: :processing)
```
Accepting a keyword argument list (`**kwargs`) results in an
`ArgumentError`
```text
ArgumentError: wrong number of arguments (given 1, expected 0)
```
