Fix requiring test_helper
Fix requiring test_helper

This was changed in https://github.com/bittisiirto/henkilotunnus/commit/d89d611ea0aa1a78b366fe22f3f4a8d4574ce259 but running `rake test` throws:
cannot load such file -- ./test_helper (LoadError)

With require ./ is used with absolute path relative to the working dir.
bittisiirto/henkilotunnus,alexhanh/hetu,alexhanh/hetu,bittisiirto/henkilotunnus