Add default_calculation_period to test runner simulation

[anth-volk]

Fixes #283. This takes the test_builder's default_period and applies as to the underlying Simulation instance's default_calculation_period. This is as simple as popping one in for the other, since they're both the same type.

This PR also adds a fail-safe whereby, if no simulation period is defined anywhere, not even as the default fallback period for the test run, the tests just use the current year. This could be removed if failure would be preferable.

[nikhilwoodruff]

Adding a test that Core doesn't break the US microsimulation (cc @MaxGhenis)

[nikhilwoodruff]

Merged through #285 - thanks!