tests: allow passing --seed <number>

This parameter is printed by every `brew tests` run and feeding it back
into the test suite allows to get an identical run. Make it easier to
do that by accepting the argument as it is printed during `brew tests`.

This requires a slight adjustment to named-argument passing as the
number that follows `--seed` is also seen as a named argument, but we
usually only want to pass `TEST=<something>` and `TESTOPTS=<something>`.