autotests: validate that KFormat does what the test expects
The test is checking for specific formatting, so before failing in the widget-text tests, validate that KFormat returns "Yesterday, ..." as text for a date that was yesterday.
It doesn't: changes in KCoreAddons were not propagated to Baloo's autotests. Repair that by doing some additional validation to explicitly match KCoreAddons behavior, and then repair the regexes used in these tests.