[backend] Updated flu epidemic method to check if epidemic is occuring during the two weeks preceding the specified date

[tests] Added edge case method to test if epidemic occurs when starting one day before date in testIsFluEpidemic() assertTrue test, and one day after date in testIsFluEpidemic() assertTrue test.

[tests] Updated testIsFluEpidemic method to reflect correct dates based on test data in influenzaEpidemic.sql file.

Updated isFluEpidemic method to check if epidemic is occurring in region for the two weeks preceding the provided date.