This approach is incompatible with the sys.path extension approach
in the next commit which is used to to find the vendored copy of
secp256k1lab, so use __file__ instead which works as well.
- added 1 more successful test vectors.
now there are 8 test vectors[test vectors 0..7].
- test vector 5 has optional message
- test vectors 5, 6, 7 have G=GENERATOR