stbt._find_tessdata_dir: add tessdata directory suffix for Fedora
stbt._find_tessdata_dir: add tessdata directory suffix for Fedora

As tested on Fedora 17, 19 and 20, the `tessdata` directory is
`/usr/share/tesseract/tessdata`, which wasn't being found by
`_find_tessdata_dir`. This commit adds the valid suffix so that, among
other things, the selftests which require this function works.

martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester