sellFYTokenPreview should return base amount, not shares amount

[marcomariscal]

these tests should be updated to not fail once this issue is fixed:

• testFailForkUnit_Euler_tradePreviewsDAI04
• testFailForkUnit_Euler_tradePreviewsUSDC04

[ultrasecreth]

I fixed this issue a while ago, but of course you'll still see the issue in the L1 dec pools as they're deployed before the fix