extract srcDir and outDir from individual test methods
[BREAKING CHANGE] fix #79 and a FIXME in the code

 - WARNING: This commit changes existing behavior and may therefore break builds using the asciidoctor plugin
   - additional validation step which checks for every file in the sourceDocumentNames if it is reachable from the sourceDir
   - use canonicalPath to compare files instead of just comparing the file name.
fix tests which were broken by the previous commit

 - the previous commit introduced a breaking change. the fix for failing tests were committed separately in this commit to emphazise this fact.