Seems that the fix for this is similar to this post: ERC20 decimals, specifying a big number - #4 by abcoathup
where I needed to use big number libraries multiply and power functions to do the calculation as the number is too big for JavaScript. The test is now working with:
const { BN, expectEvent, constants } = require('@openzeppelin/test-helpers');
const amount = new BN('100);
const price = new BN('5060584000');
const decimalsA = new BN ('5');
const decimalsB = new BN ('18');
const baseTen = new BN('10');
var totalValue = amount.mul(price);
totalValue = totalValue.mul(baseTen.pow(decimalsB.sub(decimalsA);