Add non-database unit tests for yfinance calls

[cameronstackhouse]

Summary

• Added non-database related unit tests for the finance.py module in the test_finance.py module.
• These tests can be run using pytest.
• Additional changes to the codebase involve adding a "database_path" parameter to all functions which make connection with the DB. This has been done for testing purposes so that a test DB can be created when running unit tests.

Contributes to #30.

[IsaacCheng9]

Looks like a good start to getting tests up and running!