mtr detects a forced combination if the command line for a test already
includes all options from this combination. options are stored in a perl
hash as (key,value) pairs.

this breaks if the command line has two options with the same name,
like --plugin-load-add=foo --plugin-load-add=bar, and the combination
forces plugin foo.

In particular, this resulted in warnings when running
federated.federatedx_versioning test