Fix test suite for bridgedb.scheduled.Unscheduled

Previously, the test suite checked only that Unscheduled.getInterval would
return a particular value when invoked with no argument.  Now, the test
suite checks the only property of Unscheduled.getInterval that its callers
rely on: that it returns the same string regardless of which time is passed
in to it.
Fix bug shown in test_Unscheduled_getInterval_is_constant() unittest.

 * FIXES an additional problem for #12147 pointed out by Robert Ransom's
   `bridgedb.test.test_Unscheduled_getInterval_is_constant()` unittest.
Merge branch 'rransom/fix/12147-schedule-unittest' into develop