Compare BRF in xprocspec

[josteinaj]

Moved from snaekobbi/issues#18:

Make it possible to compare BRF files in xprocspec, e.g.:

    <x:context label="brf">
      <x:document type="file" method="text" href="output-dir/file1.brf"/>
    </x:context>
    <x:expect label="brf" type="compare">
      <x:document type="file" method="text" href="../resources/file1.brf"/>
    </x:expect>

The problem is that XML 1.0 doesn't support the Form Feed character entity. You can use the binary method but this makes it difficult to compare the actual result and the expected result. Some ideas:

• Use method="binary" instead, then interpret the result as some charset.
• Add a preprocessing step that transforms a BRF into an XML structure where a Form Feed leads to a new <page>. Maybe method="brf".
• Moving on from the previous point, directly output a basic PEF where instead of Unicode braille you have ASCII braille.