use_test() creates wrong file if period in file name

I'm on a "data.frame.R" tab, it will create a "test-data.R", I expect a "test-data.frame.R" tab.

It's due to `slug()`, due in turn to `fs::path_ext_set()` but I think for the latter it's the intended behaviour.

```
usethis:::slug("data.frame.R", "R")
#> [1] "data.R"

fs::path_ext_set("data.frame", "R")
#> data.R
```