tune simulation tests to use 10^18 decimals

[mtsitrin]

currently, the simulation tests uses non EVM token decimals in some places:

• non 10^18 power reduction
• funding genesis accounts with int64 amounts
• different fees have adym notion instead of DYM

e.g x/incentives/simulation/genesis.go:

func getFee(r *rand.Rand) math.Int {
	// use comparatively small numbers as the initial account balance is always bounded with max Int64 in simulation
	w, _ := simulation.RandPositiveInt(r, commontypes.ADYM.MulRaw(100_000))
	return w
}