Leverages the FakePrefsBackend from the previous commit to change the
fiscal year for testing.
Makes now_t a defaulted parameter of gnc_relative_date_to_time64 so
that we can inject different reference dates for testing.
Only the quarter's start month in reports is adjusted when Absolute is selected in the Start Date on the Accounting Period preferences. This branch adjusts both the quarter's start day and month when Absolute is selected or when Relative and Today, Start of this month, or Start of previous month are selected.
If the accounting period start-choice is anything but absolute instead
of matching the accounting period month and day to the present month and
day. The latter is a non-obvious hack that depends on
gnc_accounting_period_fiscal_start() returning the present day if it's
not set.