Test discovery can spawn huge numbers of "dart run test:test --version" commands, consuming all RAM #5936

While investigating #5934 I decided to test with the entire Dart SDK (to get better profiling data, since the small subset is not as slow for me). My PC ground to a halt as 100% of my available RAM was consumed.

In the SDK, there are tons of projects and we end up trying to run dart run test:test --version for them all (concurrently). I'm working on a fix so that we don't do this for multiple projects in the same Pub Workspace, but since most of the projects here are not in a Pub Workspace, it won't totally solve the problem.

We need to do something better here (we're running --version to determine the capabilities, such as whether we're a new enough pkg:test to use new coverage flags.. which needs to be known to create the profile).