[systemtest] Disable OTBN test with model
[systemtest] Disable OTBN test with model

This test won't work until the Python model is discoverable in
out-of-source-tree runs.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan