test_ocr: Refactor: Use image relative paths
test_ocr: Refactor: Use image relative paths

Advantages:

* Shorter
* It doesn't assume that tests are run with cwd = stb-tester
* Emacs inline image display & path auto-completion works

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester