Fix testData Buffer rate when fixedPointOne is different than 1e18

[brunoguerios]

Example for ghUSDC on SONIC:

main token decimals -> 18
underlying token decimals -> 6
fixedPointOne -> 30 decimals
convertToAssets should be called with 1e30 to result in the correct rate of -> 1040198652125

here's the link to the rate provider