Allow tests to be run from subdirectories

This fixes a bug where currently if you try and run tests.html from a subdirectory 

e.g. running /js/third_party/papaparse/tests/tests.html 

the 2 tests above will fail.
I tried that initially but it didn't seem to work. I didn't have time to investigate it but I assumed the worker was running from SCRIPT_PATH

I tested the above in a couple of different directories and it seemed to work.