Add more prayer level tests to expose a bug in which
needed prayer levels for next combat level was 0 instead of 2 in some
corner cases and implemented fixes.
For example: Let prayer level p be an odd number. The current combat level
is exactly same for p and p - 1. Therefore given n is the needed prayer
levels for a combat level with p - 1 prayer then n - 1 is the needed
prayer levels for a combal level with p prayer.
Further more the usage of Math.floor was incorrect in every cases so it is
changed to Math.ceil.
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