Checkbox requires trailing newline when cat'ing test definition files

Checkbox test definition files are simply contain plain text using the rfc822 format. So, suites should be able to cat the content of those files in order to combine their content into one big suite. The only problem is that if the cat command is used, then it becomes a requirement for each file to contain a trailing newline. This is unfortunately error prone.

Instead, a filter should be provided which could take the rfc822 format on stdin or multiple files as arguments, and then output the content of those files with proper spacing. Furthermore, the same script could be reused to either whitelist or blacklist the content based on arbitrary key/value pairs.