Add sighash tests from data file

[maraoz]

This adds SignatureHash tests derived from a data file, instead of generating random transactions like the existing test. It allows other libraries (in my case https://github.com/bitpay/bitcore) to import the test vectors knowing they pass in Bitcoin Core. This is my first contribution (and the first C++ code I wrote), so please let me know if it needs corrections.

[maraoz]

btw: the new test data was generated using the existing previous test, by adding a few lines that printed the needed data.

[sipa]

Perhaps catch and report deserialization errors here?

[sipa]

Looks good to me. Would you mind including the code that was used to generate these tests (perhaps in an #if 0'ed block in sighash_tests.cpp)?

[laanwj]

ACK

[maraoz]

updated with fixes suggested by @sipa

[BitcoinPullTester]

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[jgarzik]

ACK