neomutt: 20200320 -> 20200417

https://github.com/neomutt/neomutt/releases/tag/20200417

To fix the tests, I had to copy the recently created
`neomutt-test-files`[1] repository into the build-environment.

Also applied a patch from master[2] which ensures that the
`change-folder` macro actually switches to the specified folder.

[1] https://github.com/neomutt/neomutt-test-files
[2] https://github.com/neomutt/neomutt/commit/9e7537caddb9c6adc720bb3322a7512cf51ab025