find_test_scripts should only strip trailing [rR] and not any [rR]

This [line](https://github.com/hadley/testthat/blob/master/R/test-files.R#L76):

```
test_names <- gsub("\\.[rR]", "", test_names)
```

Should be (I think):

```
test_names <- sub("\\.[rR]$", "", test_names)
```

I'll submit a pull request in a few minutes to fix.
