Explicitly test if 'testthat' is a directory.

Using the latest testhat and devtools from github, on R 3.0.2 on Windows 7 I get the error message "No tests found" using test_check() on Windows (and also when test_check() is called from check() in the devtools package).

This appears to be because `!file.exists("testthat/")` (with a trailing slash) evaluates as `FALSE` on Windows even when the `testthat` folder exists. `!file.exists("testthat")` evaluates as `TRUE` on Windows if the `testthat` folder exists (though of course it doesn't test if `testthat` is a folder or not. This patch fixes the problem by explicitly testing if `testthat` is a folder.