This PR fixes tests that were broken by commit 84e23047
That commit fixes the rounding of Float fields, ensuring that the
floats' 'digits' parameter is enforced.

Issue
-----
Here an issue arise with the Discount field of `account.move.line`. In
this case, the discount value is calculated from a price and a discount
amount.

Let's say we have an amount of $180 with a $30 discount amount. This
gives a discount percentage of 16.6666666667 %. As expected, we can get
the correct discounted amount like so:
`180 * (1 - 0.166666666667) = 150`

The default decimal accuracy of the discount field on
`account.move.line` is 2 (and it's unlikely to be set much higher).
Before commit 84e23047, that accuracy what not enforced in
computes/onchanges, so the test ran without failure.

But now the precision is always applied. As a result, the discount
percentage become 16.67 %. However, with this value the test fails
because, we cannot get the expected discounted amount:
`180 * (1 - 0.1667) = 149.994 = 149.99`

Fix
---
Add a context manager to be used whenever we need Discount to have as
much precision as possible. That context manager simply patches
`DecimalPrecision.precision_get` to return a discount precision of 100

closes odoo/odoo#137731

Signed-off-by: Florian Gilbert (flg) <flg@odoo.com>