Add unit tests

[glethuillier]

The current implementation does not have any unit test:

xpub-scan/package.json

Line 8 in 3b64b7f

"test": "echo \"Error: no test specified\" && exit 1"

New unit tests could notably ensure that:

• The address type (legacy, SegWit, etc.) is associated with the correct derivation
• Check that the balance associated with a "frozen" xpub is the expected one
• Exceptions are well handled
• The address matching mechanism (perfect and partial match) is working as expected
• The --import feature (automatic validation of imported CSV files) correctly identifies optimal (i.e. no discrepancy) and erroneous (missing transactions, incorrect amount, etc.) CSV files