How to re-use setups in multiple tests?

I'm wondering if there's a pattern for re-using setups across several tests, similar to how a setUp function is used in xUnit. The problem is I need the setup to actually be re-run, for each test to start from a clean slate, so using the following doesn't work as the setup is just run before the first test.

test_expect_success setup '
     # setup repostory to a particular state
'
test_expect_success test1 '
    # some test that expects the state to be as defined in setup, and
changes state of repository
'

test_expect_success test2 '
    # another test that expects the state to be as defined in setup
'

Is there a convention for doing this that's already used? Perhaps pulling the setup code into a function or duplicating the code? Or is it better to create a separate test file for tests that need to be isolated?

Thanks,

-Tom