Pkg3: Separated Test Environments

Will one be able to set a separate environment for testing? I don't think that the current REQUIRE setup is sufficient for the current organization-by-organizations setup. Sometimes there are large changes that are happening in tandem, and it would be helpful to have a designated way to say "this package needs to be on master for tests".

One case where this shows up is in organizations where there's a change to a main package which cases changes in all dependents. Another case where this shows up is where you create features which are dependent on a large change in some other package, like getting your package ready for DataFrames' nullable-array interface and wanting the proper CI. Currently this is done with little hacks around it (calling an extra script to checkout packages only when the test is run on CI), but I think making it standard and explicit would be beneficial for both developers and those who are reviewing packages (@tkelman).

Start by reading the current REQUIRE setup and the issue discussion, then examine the extra CI script used to check out packages for tests. The work is done when package-specific test environments can be declared explicitly and the CI behavior is standardized without those hacks.

Written by the indexing model from the issue text.