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>`.
Some tests, that construct a Git repository for testing, will silently
fail if the global Git user isn't properly set up. There are valid use
cases for this, thus use a fixed commit author/committer for all tests
instead of relying on the user's configuration.
