Provide an easy way to run a single e2e test

[yfyf]

Currently you can do this awkwardly by commenting out the second nix-build in ./build test-e2e and then using the test runners in ./result.

Maybe the best thing to do would be to create an e2eTest component in the top-level default.nix that hard-codes the e2e test flags and exposing the tests via a passthru, avoiding the current awkward double-nix-build invocation in ./build test-e2e and allowing to selectively run tests? Not sure, needs some thought.