[bazel] Allow opentitan_functest to accept custom bitstreams
[bazel] Allow opentitan_functest to accept custom bitstreams

Signed-off-by: Miles Dai <0447d2cf64f2d947597a59a7749da4cb3bbe9dd1@google.com>

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