.setup= argument to allow re-usable (but hermetic) local variables?

In plain {testthat} we can re-use local variables "hermetically" by running some code before expectations:

```r
test_that("a test", {
var1 <- 1
var2 <- 2

expect_equal(var1, var2 - 1)
# ... more tests using var1,var2 ...
})
```

After `"a test"` runs, we don't have to worry about other tests "seeing" `var1`/`var2`, their scope is restricted.

It's not so easy to do this with {patrick}, because we want to declare just once variables used across all the `.cases`. We might also want to declare the `.cases` themselves.

The workaround that I've landed on is to wrap `with_parameters_test_that()` with `local({})`, but this is a tad clumsy and usage of `local({})` is not so prevalent in R code, so it slightly harms readability IMO.

One solution would be to offer a `.setup` argument that encapsulates what `local()` is doing:

```r
with_parameters_test_that(
"a parametrized test",
.setup = {
var1 <- 1
var2 <- 2

cases <- expand.grid(p1 = 1:10, p2 = 10:1)
},
expect_equal(p1 + p2, 10 * var1 + var2 - 1),
.cases = cases
)
```

`.setup` is run first, so `var1` and `var2` are available to `code` _but also_ to the other arguments in the `with_parameters_test_that` call.

WDYT?