contrib: make gen_key_io_test_vectors deterministic

Also, remove instructions which are redundant with the README

contrib/testgen/README.md

@@ -2,7 +2,7 @@
 
 Utilities to generate test vectors for the data-driven Bitcoin tests.
 
-Usage:
+To use inside a scripted-diff (or just execute directly):
 
     ./gen_key_io_test_vectors.py valid 70 > ../../src/test/data/key_io_valid.json
     ./gen_key_io_test_vectors.py invalid 70 > ../../src/test/data/key_io_invalid.json