[BUGFIX] fix qgsRound for negative numbers. Fixes #20861

[lbartoletti]

Description

The round/qgsRound function returns a false result for negative numbers.

For example qgsRound( -1234.567, 2) should return -1234.57 and not -1234.56 (tested on python and postgis)

I added this test and corrected those on negative numbers.

Fixes #20861

cc @nyalldawson

Checklist

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