[Test] Unit tests for contract module

[mojoX911]

This includes some preliminary unit test cases for contract module.

Note:

• I have updated the read_locktime_from_contract() to read 8 byte i64 data, but I will cross verify with OP_CSV execution to write and read the correct data type from script.

• I have added an read_pubkeys_from_contract_reedimscript() function as a helper, kept it because could be useful in future.

• I have skipped the method calls including Wallet and Client, planning to cover them in detail in separate module tests.

Let me know if anything more needs to be added.

[chris-belcher]

Thanks for this. I've left some small comments.

[mojoX911]

Updated addressing the review comments.

Ready for next look.

[chris-belcher]

Sorry for taking so long to get back to this. One more nit left, feel free to ignore. Afterwards squish the commits into one with a commit message following these rules: https://chris.beams.io/posts/git-commit/

[mojoX911]

Thanks for the article. Really helpful. Updated as requested. Rebased and squashed.