Add a way to (also) run tests with all packages loaded / with no packages loaded?

[fingolfin]

Some problems only appear if all packages are loaded. Perhaps we should add an option (activated via some env var, say TEST_WITH_LOADALLPACKAGES=yes) which runs LoadAllPackages() before running the test suite, so that package authors can easily add such a test job to their CI setup.

For this to work resp. actually load "most" packages, it may be necessary to add a few packages to the list of packages to build.