move marketvolume_test to colocate with its package

[ajlacey]

move test from backend/tests/market_math/marketvolume_test.go to the package it is testing to handlers/math/market package here: backend/handlers/math/market/marketvolume_test.go

This test uses a table-driven test format.
A common convention for these is to name the table tests or testCases, when using this convention it is best to use test or tc respectively in the loop here: https://github.com/openpredictionmarkets/socialpredict/blob/main/backend/tests/market_math/marketvolume_test.go#L69-L76
Update this loop to follow this convention during the move.
Add a test description (comment) here: https://github.com/openpredictionmarkets/socialpredict/blob/main/backend/tests/market_math/marketvolume_test.go#L9

[snagai]

Hello @ajlacey
May I take this?