GitHub Actions: Migrate specialized tests previously on Travis CI to GitHub Actions

[HenrikBengtsson]

Travis CI is no longer free. Migrate the specialized checks there to GitHub Actions;

https://github.com/HenrikBengtsson/future/blob/8409464f189a8fbeced3f8ae40a113feebe8521f/.github/.travis.yml#L28-L102

[HenrikBengtsson]

Env vars

• [x] R_FUTURE_FORK_ENABLE=false
• [x] R_FUTURE_PSOCK_RELAY_IMMEDIATE=false
• [x] R_FUTURE_PLAN=multisession, R_FUTURE_FORK_MULTITHREADING_ENABLE=false
• [x] R_FUTURE_GLOBALS_ONREFERENCE=warning, R_FUTURE_RNG_ONMISUSE=error

Using future.tests

• [x] sequential
• [x] multisession
• [x] cluster
• [x] multicore

Example:

R CMD build --no-build-vignettes --no-manual . 
R CMD INSTALL *.tar.gz 
Rscript -e future.tests::check --args --test-plan=sequential