Confusing display of command-line options for merged run sets

[PhilippWendler]

When merging run sets with a <union> tag, the table with the benchmark environment contains all the individual values for run-set names, command-line options, etc. separated by ;, like this:

This is correct but not easy to read, especially for the command-line options. We should at least reformat the latter, for example by using a proper HTML list instead of values separated by ;. That should not be difficult.

Even better would be a way that would allow readers to determine easily which run set has which options instead of having to map the first run set with the first set of options, but this is more complicated and unlikely to be implemented soon. Ideas on how this could be represented would be welcomed, though.

Issue based on https://github.com/sosy-lab/benchexec/issues/542#issuecomment-1054979650 by @shonfeder.