this opts out of all four test files unless opted in

[eddelbuettel]

This PR is a little radical in that all tests are now conditional on being explicitly opted in via a new environment variable.

This may help at CRAN. The curl issue seems random and even hit me here but was easily fixed by a re-run. This may be an issue in curl, seen "randomly" at at least the CRAN Debian machine and here under ubuntu-latest.

I am wondering if you should add a simple "lighter" unit test file with something not involving a curl call so that we always have "something" to run?

Also, if you merge the previous PR I can rebase it in here so that this PR gets the benefit of the tests.

[PMassicotte]

I like the way you implemented it. I also ran into random errors, even on my own machine.

I am wondering if you should add a simple "lighter" unit test file with something not involving a curl call so that we always have "something" to run?

Are you thinking about something specifically? I can not think to a test not involving curl right now. I will open an issue where I will trow out ideas when they come.