Add `JULIA_REFERENCETESTS_UPDATE_FORCE` for updating even non-failing tests

[IanButterworth]

Sometimes you want to update references with subtle changes less than the psnr threshold.

JULIA_REFERENCETESTS_UPDATE=true only updates failing ones.

[oxinabox]

Right now you can do this via deleting them.
Then run with JULIA_REFERENCETESTS_UPDATE=true
Maybe that is enough?