How to configure OpenVAS where it is NOT locking out "root" accounts?

The following comments have some background info why you shouldn’t do any changes on scanner side:

TLDR; Don’t weaken the scanner and instead configure the target device to not lock accounts in general but only based on the source IP.

If you still want to accept the risk of missing detection of weak / hardcoded default credentials (which is strongly discouraged) the second comment includes some additional info.