MDL-18375 calendar: removed uses of PHP date functions not present on all PHP installs

Changed the logic of the test calendar so that it does not use the
Julian date calendar as the PHP date conversion functions (eg. jdtogregorian)
are not always present on all PHP installs. Also split the unit test
into multiple tests and rearranged the order parameters were passed to
certain functions as they were changed in the previous commit.