[JENKINS-18154] Refactor getDoxygenGeneratedDir() to use FilePath

Using the FilePath abstraction instead of concatenating path strings
simplifies the logic substantially; it no longer matters whether the
path is relative or absolute, for example.
[JENKINS-18154] remove isDirectoryAbsolute()

The refactor of getDoxygenGeneratedDir() has made isDirectoryAbsolute()
redundant; remove it.
[JENKINS-18154] Add test for OUTPUT_DIRECTORY=.