Add integration tests for remote package restore

[ChaitaC]

Context

Posit Connect has been removing its integration tests that verify restoring packages from remote sources (posit-dev/connect#39649, posit-dev/connect#39655). Testing remote download behavior is more suited to Packrat itself since it owns that logic.

Proposal

Add integration tests that verify packrat::restore() works with remote sources. Modeled on how renv does it (test-retrieve.R):

1. Create a minimal test package hosted under a Posit-controlled account/org
2. Write testthat tests that restore from a lockfile pointing to the remote source
3. Assert the package installs and can be loaded
4. Mark as skip_on_cran() / skip when offline

Open questions

• Should the test package live under rstudio/ on GitHub? Elsewhere?
• Should these be opt-in (skipped by default) given they require network access?

References

• Connect PR removing Bitbucket test: posit-dev/connect#39649
• Connect issue removing GitLab test: posit-dev/connect#39655
• renv's approach: https://github.com/rstudio/renv/blob/main/tests/testthat/test-retrieve.R