Don't auto-switch to root when executing make-container.sh, because
auto root switching is also implemented in extra-container.
Besides simplifying the code, this is useful for a later commit that
introduces flakes-based container building.
With this change, the container is built under the regular user
instead of root, thereby utilizing the user's regular fetcher and
evaluation caches.
The result of `import tests.nix {}` is now an attrset of tests.
This makes it easier and more efficient to evaluate or build multiple
tests in one call to `nix build`.
Simplify tests.nix by removing the large module args scope in favor of
self-contained scenario module definitions.
Add CPU core and memory size defaults to the test configuration to
enable building tests without `run-tests.sh`.
Add the following top-level args to tests.nix:
- `extraScenarios` to provide a nix-level way to define extra scenarios.
- `pkgs` to allow building tests with custom pkgs or systems.
