CI: API hearth beat are not reporting errors

It's because:

$ R -q -e 'testthat::test_package("RADIS", stop_on_failure = TRUE)'
> testthat::test_package("RADIS", stop_on_failure = TRUE)
> 
No installed testthat tests found for RADIS

The subtil reason is because pak::local_install() does not install tests by default. This is a regression introduced in 582cb2d7.