Use deep cloning of reporters in auto_test() and auto_test_package()

[krlmlr]

Fixes #441. CC @nealrichardson.

Also uses withr::with_envvar() instead of devtools::with_envvar() to avoid warning.

Testing auto_test() and auto_test_package() is tricky, but not impossible. An external R script that runs for a certain time can be started using a pattern like this:

system("Rscript -e 'Sys.sleep(Inf)' & sleep 1")

The output of that script could be then analyzed by the test.

NEWS entry:

- `auto_test()` and `auto_test_package()` show only the results of the current test run and not of previously failed runs (#456, @krlmlr).

[hadley]

Merged manually