adds skip_on_gh_actions (resolves #970)

[njtierney]

Completely stolen the code from #970, but renamed function to skip_on_gh_actions() instead of skip_on_actions() - please feel free to change!

Thanks!

[jimhester]

LGTM

[hadley]

Can you please merge/rebase and move the NEWS bullet up?

[coatless]

@njtierney outside of rebasing, could you add the skip_on_gh_actions() function to the details section of the documentation so that it is alongside the other skip variants? (I stumbled across this looking for the relevant skip_on_*().

https://github.com/r-lib/testthat/pull/975/files#diff-8118c4bf22b8b996a3b1d49ba32e01feR23-R30

#' `skip_on_ghactions()` skips tests on GitHub Actions by inspecting the
#' `GITHUB_ACTIONS` environment variable.
#'

[hadley]

Closing since you can just use skip_on_ci(), which is more general.

[kevinushey]

Does skip_on_ci() need to check GITHUB_ACTIONS as well? It seems like CI is not defined on GitHub Actions; at least as documented at https://help.github.com/en/actions/configuring-and-managing-workflows/using-environment-variables.

[hadley]

We set it in our CI actions, which I think will cover 99% of R users.