Write package test tempfiles inside the session temporary directory

Message from CRAN:

```
Dear maintainer,

Checking this creates

  /tmp/.checkpoint

as you do

checkpointLocation = dirname(tempdir())
dir.create(file.path(checkpointLocation, ".checkpoint"), showWarnings = FALSE)

Can you pls change to use a location inside the session directory?
```

The fix is to use 

``` r
checkpointLocation = tempdir()
```
