Split serialization tests into staging and regression

[rmgaray]

Depends on #24 .

Description from latest commit:

Manage staging and regression tests separately

This commit refactors the serialization tests
to handle staging and regression tests separately.

TXs in the staging folder that are also available
in the regression folder are automatically
filtered out, since they will be tested anyway
when the regression suite is run.

In practice, this means that one should run the
tests twice after adding new TXs to staging
to get a stable output report.

This commit also improves the report to include
failure messages.

To get new transactions from the blockchain tip, run npm run get-transactions. This will automatically delete all TXs saved under staging and replace them with new ones.